what is powerwall？what are we doing？

What is powerwall？

Powerwall is a rechargeable lithium-ion battery designed to store energy in residential buildings. It will realize load transfer, power backup and solar self-sufficiency. Work together with photovoltaic panels and inverters. During the day, photovoltaic power generation is used to provide energy for the family, and excess energy is stored in the energy wall. When there is no sun at night, the powerwall starts to supply power for the family, and it will not connect to the national grid or choose to charge the powerwall until the power is exhausted.Powerwall is an energy storage system built for families to meet your daily electricity needs.

How does powerwall work？

Powerwall can charge at a low price when the power demand is low, and output electric energy during the peak time when the power price is higher. Powerwall can increase the capacity used by household solar energy, and provide power backup guarantee when the power grid is interrupted.

Why need powerwall？

When the cold season comes, due to heavy snow and low temperature, the power is often cut off, and some areas are completely cut off. Put those who try to keep warm by all possible means at risk. However, when you install the powerwall, you can avoid the cold pain of power failure, and it will store energy to provide you with warmth. Having your own energy system can protect you from the most unpredictable situations.

What about our powerwall？

Our powerwall has designed 5/6/9/10/13kwh, five capacities can be selected, and customers can choose to purchase according to their household electricity demand and your system budget. At the same time, we also provide customized services.

Like all our battery systems, we have done a lot of tests on powerwall and built in security measures. Like all electronic devices, the energy wall is UL certified. (Background information: American Underwriters Testing Institute is a safety certification agency, which publishes standard tests and sometimes participates in the design of electrical safety. All electrical equipment has a UL certification mark. Without this mark, insurance companies will not provide insurance for large products, and regulators (generally) will not allow the sale of consumer goods.)

 

 

Synthesis of Alumina-Coated Natural Graphite for High Cycling Stability and Safety of Li-ion Batteries

Research Background：

Al ₂ O ₃ in lithium-ion batteries has a long history, and the most famous one is that the separator surface is coated with Al ₂ O ₃ to improve the thermal stability of the separator, and the aluminum oxide coating on the negative electrode of Samsung SDI is proposed by Dr. Zhang Zhengming of Celgard. Overlay improves battery performance and safety. However, there are more and more studies on the application of Al ₂ O ₃ in lithium batteries recently. A little further, the research of the research group of Liu Zhaoping from the Ningbo Institute of Materials, Chinese Academy of Sciences shows that adding silane-Al ₂ O ₃ to the conventional electrolyte can not only improve the electrochemical performance of the battery such as cycle and rate, but also effectively improve the safety of the battery. Then, the research of Dae Sik Kim et al. of Korea showed that the graphite surface coated with amorphous Al ₂ O ₃ can improve the fast charging performance of the battery. Recently, Jeff Dahn put forward a new insight that Al ₂ O ₃ coated on the surface of NCM can react with LiPF ₆ to form LiPO ₂ F _{2 ,} which can improve battery performance.

Due to the trend of various factors, the capacity and energy density of power batteries continue to increase, and the safety of batteries is increasingly concerned. The safety issues of batteries are complex, but one of the root causes is the chemical system. Only by fundamentally improving the safety of materials can it be possible to improve the overall safety characteristics of batteries. Headquartered in Shenzhen, Battery has a good reputation in the field of anode materials, and its R&D strength is second to none among domestic material factories. Recently, Dr. Xu Tao, who specializes in improving the safety of negative electrodes, proposed that Al ₂ O ₃ can be coated on the surface of natural graphite by the sol-gel method, which can not only improve the cycle stability of the battery, but also improve the safety. Alumina-Coated Natural Graphite for Highly Cycling Stability and Safety of Li-Ion Batteries was published in Chinese Journal of Chemistry.

Analysis of pictures and texts：

Figure 1. (ac) Natural graphite (NG), 1 wt% Al2O3-coated natural graphite ( denoted as AN-1), and 3 wt% Al2O3-coated natural graphite (denoted as AN-1 ).

Figure 2. (a) and (e) are the SEM images of AN-1 and AN-3, respectively; (bd) and (fh) are the elemental distribution images of AN-1 and AN-3.

First, the authors coated Al ₂ O ₃ on the surface of natural graphite (NG) by sol-gel method . The preparation method of Sol-gel is very simple. First , add deionized water to Al(NO ₃ ) ₃ ·9H _{2 O}, then add natural graphite under stirring conditions, and finally dry to obtain natural graphite AN coated with Al ₂ O ₃ on the surface_. -1 (alumina coating amount of 1 wt%) and AN-3 (alumina coating amount of 3 wt%). The SEM images of NG, AN-1 and AN-3 are shown in Figure 1, and the particle sizes of the three are about 11.55 μm, 11.58 μm and 11.67 μm, respectively. It can be seen from Figure 1 and Figure 2 that the Al ₂ O ₃ coated on the surface of natural graphite by the sol-gel method is relatively uniform, but the natural graphite particles do not achieve complete coating, but there is a considerable part of the exposed area .

Figure 3. XRD patterns and TGA patterns of NG, AN-1 and AN-3.

The XRD patterns of AN-1 and AN-3 are almost the same as those of NG, and do not show the existence of Al ₂ O ₃ , which is mainly caused by too little coating of Al ₂ O _{3 .} But the TGA curve can clearly see that NG burns completely at 900 ℃, while AN-1 and AN-3 still have quality, which is consistent with the amount of Al ₂ O ₃ in them_.

Figure 4. Charge-discharge curves of NG, AN-1 and AN-3 at 0.005-3 V and 0.1 C.

Figure 5. Charge-discharge curves of NG, AN-1 and AN-3 at 3.0-4.35 V and 1 C.

Subsequently, the authors conducted a comparative analysis of the electrochemical properties of NG, AN-1 and AN-3. First, in terms of discharge capacity, the discharge capacities of NG, AN-1, and AN-3 are 364.0, 359.8, and 350.4 mA/g, respectively, and the Coulombic efficiencies are 93%, 93.4%, and 93.5%, respectively. For the reduction of discharge capacity by Al ₂ O ₃ coating, the author believes that the coating leads to the reduction of lithium ion diffusion channels; while the increase in Coulomb efficiency is caused by the reduction of side reactions by coating ( Note: the most conventional explanation is not good. Can’t say it’s bad either! )

Figure 6. (a)-(c) are the results of acupuncture experiments for 2 Ah LCO pouch cells using NG, AN-1 and AN-3 graphites, respectively.

 

The highlight is here. Finally, the author conducted acupuncture experiments on soft-pack batteries using NG, AN-1 and AN-3 three kinds of graphite, and compared the modification of battery safety by Al ₂ O ₃ coating. The needle diameter is 3 mm, the taper angle is 60 degrees, and the needling speed is 80 mm/s. As shown in Figure 7, thermal runaway occurred rapidly during the needling process of pouch battery using NG, and the maximum temperature of thermal runaway exceeded 600 °C; while the pouch battery using AN-1 and AN-3 did not experience thermal runaway during the needling test. The maximum temperature of the battery surface does not exceed 100 ℃, and the maximum temperature reached by AN-3 is lower than that of AN-1. The above acupuncture comparison experiments show that the surface Al ₂ O ₃ coating of natural graphite can definitely improve the safety of the battery to a certain extent.

summary:

In this paper, the surface of natural graphite is coated with Al ₂ O ₃ by sol-gel method_{. The} coating can not only inhibit the side reaction, improve the cycle stability of the battery, but also improve the safety. The coating amount of Al ₂ O ₃ is 1 wt% of natural graphite, and thermal runaway does not occur under the acupuncture experiment. Although the impact factor of this article is not high, it is still commendable to publish papers in enterprises, and the final battery safety improvement results are also very convincing.

 

 

 

 

 

 

Thermal runaway and fire behavior of large lithium iron phosphate batteries

Research Background

Lithium-ion batteries have been widely used due to their high energy density and good cycle performance. In recent years, the capacity of batteries has gradually increased. Understanding the thermal runaway characteristics and fire behavior of large-scale lithium-ion batteries is important for their fire prevention and control. significance. In this study, a 326Ah large-scale lithium iron phosphate battery was selected, and a series of combustion experiments were carried out to systematically study the combustion process, flame shape, thermal runaway expansion characteristics, the effect of flame on the thermal runaway process of the battery, and the law of combustion heat production.

Work introduction

Wang Qingsong’s research group from the University of Science and Technology of China selected 326Ah large-scale lithium iron phosphate batteries and carried out a series of combustion experiments, revealing the combustion characteristics of large-scale lithium-ion power batteries and filling the gap in the industry. When the battery is locally heated, based on the temperature of each surface of the battery, the propagation process of thermal runaway inside the single battery can be clearly observed, and the larger the battery size, the more obvious this process is. The combustion and mass loss process of large batteries can be divided into multiple stages. In the most violent thermal runaway stage, the combustibles in the battery are rapidly consumed, forming a violent columnar jet fire; flame combustion can cause thermal runaway of the battery to occur earlier, but the The battery surface temperature peaks have little effect. Studies have shown that compared with other low-capacity single cells, the tested large single cells not only have higher energy density, but also have smaller combustion heat per unit capacity. The results were published in the journal Renewable and Sustainable Energy Reviews . Doctoral student Mao Binbin is the first author of the paper, and Wang Qingsong is the corresponding author.

 

Description of content

Due to its extremely high safety and good cycle performance, lithium iron phosphate batteries have been widely used in electric vehicles and energy storage power stations. The research object of this paper is a prismatic lithium iron phosphate battery, the positive electrode material is lithium iron phosphate, and the negative electrode material is graphite, and its combustion fire characteristics are studied, which can provide data support for the safety early warning and fire extinguishing design of the subsequent battery system.

Based on the ISO9705 full-size room combustion test bench and the ISO5660 cone calorimeter and other combustion instruments, a medium-sized battery combustion chamber with a size of 1.8m×1.8m×2m was designed and built. In the combustion experiment chamber, the largest side of the battery was heated by a heating plate to excite it to thermal runaway combustion; the combustion phenomenon was recorded by DV, and the temperature and voltage changes were monitored by thermocouple and charge-discharge cycler respectively; based on gas analyzer measurement Based on the oxygen consumption method, the battery combustion heat release rate (HRR) was obtained.

Figure 1 shows the temperature, mass, voltage and heat release rate curves of a fully charged battery during the combustion process. Since the battery cannot spontaneously ignite, the battery is manually ignited after the pressure relief valve is opened and the electrolyte is leaked. Based on the pressure relief valve opening event and the battery quality curve, the battery combustion process can be divided into four stages. Figure 2 shows the battery combustion phenomenon. The combustion process of the 326Ah large-scale power battery is very violent. In the stage III, when the battery is rapidly thermally runaway, a large amount of combustible gas is ejected, forming a very violent columnar flame, as shown in Figure 2(g).

Figure 2. The combustion process of a fully charged battery, with the heating start time as time 0, the pressure relief valve is opened at 1459s, the battery is manually ignited at 1484s, and the flame at 2214s is a columnar intense combustion flame.

The thickness of this battery sample is 7 cm; based on the temperature curves of the heated surface, side center and back surface of the battery, a very obvious thermal runaway expansion process is observed. This study also carried out two experiments without artificial ignition to analyze the effect of flame combustion on the thermal runaway history of the battery; the results show that flame combustion can advance the thermal runaway time point of the battery, but it has an impact on the maximum temperature of the battery surface after thermal runaway. smaller.
In this study, the combustion heat production of the tested 326Ah large-scale lithium iron phosphate battery was compared with the small-capacity battery studied by the predecessors, as shown in Table 1. The 326Ah large battery in this study has the highest specific capacity, and its normalized peak HRR and combustion heat per unit capacity are relatively small, showing superior thermal safety. In addition, after removing the highest and lowest values, the average mass loss ratio of each battery was 24.5%±3.1%, and the normalized average total combustion heat was 19.9±4.9MJ kg ^-1 , showing good regularity. This shows that, based on the industrial database and experimental experience, after estimating the mass ratio of the combustibles in the battery and the combustion heat lost per unit mass, the combustion heat of the battery can be predicted only by weighing the initial mass of the battery. The above empirical formula can avoid dangerous and costly combustion experiments, and has important guiding significance for the safety assessment of lithium-ion batteries.

Conclusion

 This paper studies the combustion characteristics of a 326Ah large lithium iron phosphate power battery , which fills the gap of the current lack of combustion characteristics of lithium-ion batteries above 100 ampere-hours. The core conclusions are as follows:
1) The combustion process of this large battery can be divided into 4 stages. Stage III is the most violent thermal runaway stage. It can be observed that Violent columnar jet fire, the peak combustion heat release rate can reach 88.78kW;
2) When the battery is locally heated, there is a thermal runaway expansion process inside the battery cell, and the larger the battery volume, the more obvious this phenomenon is;
3) Combustion The flame can accelerate the spread of the thermal runaway of the battery, but it has little effect on the highest temperature on the battery surface;
4) This paper compares the combustion heat of the tested large single battery with the previous small battery, and the results show that the large single battery It has a high mass specific capacity, and its unit combustion heat production is low (the heat production per unit capacity and the peak heat release rate per unit surface area are both low); this shows that from the perspective of combustion heat production, the large-scale single cell Chemicalization is also one of the favorable directions for the development of the battery industry. Binbin Mao, Chaoqun Liu, Kai Yang, Shi Li, Pengjie Liu, Mingjie Zhang, Xiangdong Meng, Fei Gao, Qiangling Duan, Qingsong Wang* , Jinhua Sun. Thermal runaway and fire behaviors of a 300 Ah lithium ion battery with LiFePO ₄ as cathode, Renewable and Sustainable Energy Reviews , 2021, DOI:10.1016/j.rser.2021.110717

Quantitative Analysis of Expansion of Different Proportions of Silicon Carbon Anode Plates

In 2020, DanielP. Abraham’s team used an in-situ electrochemical cell to characterize the thickness expansion of graphite and silicon anodes, quantitatively analyzed the difference in electrode expansion when different proportions of silicon-carbon were mixed, and analyzed several factors that affect the in-situ expansion test. Reference for subsequent researchers.

Sample preparation and testing equipment

Different test electrode sheets were prepared according to the proportions in Table 1 for the assembly of the buckling and in situ electrolysis cells. The device shown in Figure 1 was used to measure the expanded thickness of different pole pieces.

Table 1. Composition and performance parameters of test electrodes

Figure 1. Schematic diagram of the electrochemical expansion test structure

Result analysis：

It can be seen from the expansion test curve of the negative pole piece with 15% silicon mass percentage in Figure 2 that with the progress of the cycle, the maximum expansion thickness of the fully intercalated lithium state increases significantly, while the irreversible thickness of the fully delithiated state in each cycle. The expanded thickness is also gradually increasing, which may originate from the SEI film formation and the degree of lithium deintercalation in the anode.

Figure 2. Expansion curve of silicon-carbon composite anode

It can be seen from the expansion and voltage loops of silicon carbon anodes with different ratios in Fig. 3 that for pure graphite anodes, the thickness expansion mainly occurs in the first and third platforms, while the expansion ratio of the second platform is only 1 %, and during high-capacity delithiation, the thickness will shrink to a certain extent. For the pure silicon negative electrode, the maximum expansion thickness is almost 300%, and the irreversible expansion thickness is also close to 50%. After one cycle of charge and discharge, the capacity of the battery is also the largest.

Figure 3. Potential and expansion curves of silicon-carbon composite anodes with different ratios

From the differential capacity curve in Figure 4, it can be seen that the pure graphite electrode has three obvious reaction peaks when intercalating lithium, while the peak positions of the two lithium-silicon alloys of the pure silicon electrode are larger than those of graphite. A mixture of lithium intercalation peaks for the two materials is exhibited, but the reaction peak for the lithium-silicon alloy is weaker.

Figure 4. Differential capacity curves of silicon-carbon composite anodes with different ratios

Figure 5 shows that when different proportions of silicon carbon are combined, the specific capacity of the electrode increases gradually, and the maximum expansion thickness corresponding to the fully intercalated lithium state of the electrode also increases gradually, but not linearly. When the content is less than 30%, the expansion thickness of the electrode increases slightly, but when the content is greater than 30%, the slope of the thickness expansion increases significantly.

Figure 5. Specific capacity and maximum expansion ratio of silicon-carbon composite anodes with different ratios

Figure 6 shows the three influencing factors of the electrochemical expansion test device: electrolyte, test pressure, and electrode porosity. The authors found that because the sealing performance of the electrochemical in-situ cell is not as good as the charge-deduction, the potential capacity of the two structures There will be differences in the curves, but the influence can be weakened by selecting LiFSi electrolyte and reducing the test magnification. Meanwhile, the porosity of the electrode will also affect the change trend of the expanded thickness during the lithium deintercalation process of the electrode. In addition to the above three, the authors also consider the influence of displacement sensor drift, test applied pressure, and gas production on the measurement of expanded thickne.

Figure 6. Three influencing factors of electrochemical expansion test device: electrolyte, test pressure, electrode porosity

Summarize

In this paper, an in-situ electrochemical cell was used to characterize the thickness expansion of graphite and silicon anodes, and quantitatively analyze the difference in electrode expansion when different proportions of silicon and carbon were mixed. The main conclusions are as follows:

1. The maximum expansion of pure graphite pole piece is ~19%, while the maximum expansion of pure silicon pole piece is ~300%;
2. The expansion ratio of silicon-containing pole pieces will be affected by the porosity of the pole piece design and may be higher than expected;
3. For different proportions of silicon carbon anodes, the expansion is nonlinear;
4. Limiting the capacity of the silicon carbon anode can adjust the maximum expansion ratio;
5. Limiting the lithium intercalation depth of the pole piece is better than limiting the delithiation depth in inhibiting expansion;
6. The expansion ratio of the pole piece with low porosity is larger than that of the pole piece with high porosity;
7. Researchers should pay attention to the influence of several factors mentioned in the text when using electrochemical in-situ expansion devices;

References

Andressa Y. R. Prado, Marco-Tulio F. Rodrigues,Stephen E. Trask, Leon Shaw and Daniel P. Abraham, Electrochemical Dilatometryof Si-Bearing Electrodes: Dimensional Changes and Experiment Design, Journal ofThe Electrochemical Society, 167(2020) 160551.

Factors affecting the internal resistance of lithium-ion batteries

With the use of lithium batteries, the performance of the battery is constantly attenuated, which is mainly manifested in capacity attenuation, increase in internal resistance, and decrease in power. Therefore, the factors affecting the internal resistance of the battery are expounded in combination with the battery structure design, the performance of raw materials, the process technology and the use conditions.

1.Structural Design Impact

In the battery structure design, in addition to the riveting and welding of the battery structure itself, the number, size, and position of the battery tabs directly affect the internal resistance of the battery. To a certain extent, increasing the number of tabs can effectively reduce the internal resistance of the battery. The position of the tab also affects the internal resistance of the battery. The internal resistance of the wound battery with the tab position at the head of the positive and negative pole pieces is the largest. Compared with the wound battery, the laminated battery is equivalent to dozens of small batteries connected in parallel. , its internal resistance is smaller.

 

2.Influence of raw material properties

(1)Positive and negative active materials –In lithium batteries, the positive electrode material is the lithium storage side, which determines the performance of the lithium battery. The positive electrode material mainly improves the electronic conductivity between particles through coating and doping. For example, the strength of the PO bond is enhanced after doping with Ni, the structure of LiFePO4/C is stabilized, and the unit cell volume is optimized, which can effectively reduce the charge transfer resistance of the cathode material. The large increase of activation polarization, especially the activation polarization of the negative electrode, is the main reason for the serious polarization. Reducing the particle size of the anode particles can effectively reduce the anode activation polarization. When the anode solid phase particle size is reduced by half, the activation polarization can be reduced by 45%. Therefore, in terms of battery design, research on the improvement of the positive and negative electrode materials themselves is also essential.

(2)Conductive agent –Graphite and carbon black are widely used in the field of lithium batteries because of their good properties. Compared with the graphite-based conductive agent, the battery with carbon black-based conductive agent added to the positive electrode has better rate performance, because the graphite-based conductive agent has a flaky particle morphology, which causes a large increase in the pore tortuosity at high rates, and is prone to Li liquid phase diffusion. Process limiting the phenomenon of discharge capacity. The internal resistance of the battery added with CNTs is lower, because the fibrous carbon nanotubes are in line contact with the active material compared to the point contact between graphite/carbon black and the active material, which can reduce the interface impedance of the battery.

(3)current collector –Reducing the interface resistance between the current collector and the active material and improving the bonding strength between the two are important means to improve the performance of lithium batteries. Coating conductive carbon coating on the surface of aluminum foil and corona treatment of aluminum foil can effectively reduce the interfacial impedance of the battery. Compared with ordinary aluminum foil, the use of carbon-coated aluminum foil can reduce the internal resistance of the battery by about 65%, and can reduce the increase in the internal resistance of the battery during use. The AC internal resistance of corona-treated aluminum foil can be reduced by about 20%. In the commonly used SOC range of 20% to 90%, the DC internal resistance is generally small and the increase is gradually smaller with the increase of the discharge depth.
Diaphragm–The ion conduction inside the battery depends on the diffusion of Li ions in the electrolyte through the pores of the separator. The liquid absorption and wetting ability of the separator is the key to the formation of a good ion flow channel. When the separator has a higher liquid absorption rate and porous structure, it can be improved. Conductivity reduces battery impedance and improves battery rate capability. Compared with ordinary base film, ceramic diaphragm and glued diaphragm can not only greatly improve the high temperature shrinkage resistance of the diaphragm, but also enhance the liquid absorption and wetting ability of the diaphragm. Adding SiO2 ceramic coating on the PP diaphragm can make the liquid absorption of the diaphragm. volume increased by 17%. Coating 1μm PVDF-HFP on the PP/PE composite separator, the liquid absorption rate of the separator increased from 70% to 82%, and the internal resistance of the cell decreased by more than 20%.

In terms of process technology and use conditions, the factors that affect the internal resistance of the battery mainly include:

3.the influence of process factors

(1)slurry dispersion during slurry mixing affects whether the conductive agent can be uniformly dispersed in the active material and is in close contact with it, which is related to the internal resistance of the battery. By increasing the high-speed dispersion, the uniformity of slurry dispersion can be improved, and the internal resistance of the battery is smaller. By adding surfactant, the distribution uniformity of the conductive agent in the electrode can be improved, and the electrochemical polarization can be reduced and the discharge median voltage can be improved.

(2)Coating surface density is one of the key parameters of battery design. When the battery capacity is constant, increasing the electrode surface density will inevitably reduce the total length of the current collector and the separator, and the ohmic internal resistance of the battery will decrease accordingly. Therefore, within a certain range , the internal resistance of the battery decreases as the areal density increases. The migration and detachment of solvent molecules during coating and drying are closely related to the temperature of the oven, which directly affects the distribution of the binder and conductive agent in the pole piece, which in turn affects the formation of the conductive grid inside the pole piece. Temperature is also an important process for optimizing battery performance.

(3)Rolling To a certain extent, the internal resistance of the battery decreases with the increase of the compaction density, because the compaction density increases, the distance between the raw material particles decreases, the more contacts between the particles, the more conductive bridges and channels, the more the battery Impedance decreases. The control of the compaction density is mainly achieved by rolling the thickness. Different rolling thicknesses have a greater impact on the internal resistance of the battery. When the rolling thickness is large, the contact resistance between the active material and the current collector increases due to the failure of the active material to be rolled tightly, and the internal resistance of the battery increases. And after the battery is cycled, the surface of the positive electrode of the battery with a larger rolling thickness will have cracks, which will further increase the contact resistance between the active material on the surface of the pole piece and the current collector.

(4)Pole Piece Turnaround Time Different shelving time of the positive electrode sheet has a great influence on the internal resistance of the battery. When the shelving time is short, the internal resistance of the battery increases slowly due to the interaction between the carbon coating on the surface of the lithium iron phosphate and the lithium iron phosphate. When the shelving time is longer (more than 23h), the internal resistance of the battery increases significantly due to the combined effect of the reaction between lithium iron phosphate and water and the bonding effect of the binder. Therefore, the turnaround time of the pole piece needs to be strictly controlled in actual production.
Injection The ionic conductivity of the electrolyte determines the internal resistance and rate characteristics of the battery. The conductivity of the electrolyte is inversely proportional to the viscosity range of the solvent, and is also affected by the concentration of lithium salt and the size of the anion. In addition to the optimization study of conductivity, the amount of liquid injection and the soaking time after liquid injection also directly affect the internal resistance of the battery. A small amount of liquid injection or insufficient soaking time will cause the internal resistance of the battery to be too large, thus affecting the battery capacity play.

4.Conditions of use affect

(1)Temperature The effect of temperature on the internal resistance is obvious. The lower the temperature, the slower the ion transmission inside the battery, and the greater the internal resistance of the battery. The battery impedance can be divided into bulk impedance, SEI membrane impedance and charge transfer impedance. The bulk impedance and SEI membrane impedance are mainly affected by the ionic conductivity of the electrolyte, and the change trend at low temperature is consistent with the change trend of the electrolyte conductivity. Compared with the increase of bulk impedance and SEI film resistance at low temperature, the charge reaction impedance increases more significantly with decreasing temperature. Below -20°C, the charge reaction impedance accounts for almost 100% of the total internal resistance of the battery.

(2)SOC When the battery is in different SOC, its internal resistance is also different, especially the DC internal resistance directly affects the power performance of the battery, which in turn reflects the battery performance of the battery in the actual state: the DC internal resistance of the lithium battery varies with the depth of discharge DOD of the battery In the 10%~80% discharge range, the internal resistance is basically unchanged, and generally the internal resistance increases significantly at a deeper discharge depth.

(3)Storage As the storage time of the lithium-ion battery increases, the battery continues to age, and its internal resistance continues to increase. Different types of lithium batteries have different degrees of internal resistance change. The internal resistance increase rate of LFP battery is higher than that of NCA and NCM batteries after a long storage period of 9-10 months. The rate of increase in internal resistance is related to storage time, storage temperature and storage SOC.

(4)Cycling Whether it is storage or cycling, the effect of temperature on the internal resistance of the battery is consistent. The higher the cycle temperature, the greater the increase rate of internal resistance. Different cycle intervals have different effects on the internal resistance of the battery. The internal resistance of the battery increases rapidly with the increase of the depth of charge and discharge, and the increase of the internal resistance is proportional to the increase of the depth of charge and discharge. In addition to the influence of the depth of charge and discharge in the cycle, the charge cut-off voltage also has an influence: too low or too high upper limit of the charge voltage will increase the interface impedance of the electrode, and the passivation film cannot be formed well under the too low upper limit voltage, and Too high upper voltage limit will lead to the oxidative decomposition of electrolyte on the surface of LiFePO4 electrode to form products with low conductivity.

(5)Other Vehicle lithium batteries will inevitably experience poor road conditions in practical applications, but the study found that the vibration environment of lithium batteries during application has little effect on the internal resistance of lithium batteries.

Outlook

Internal resistance is an important parameter to measure lithium-ion power performance and evaluate battery life. The larger the internal resistance, the worse the rate performance of the battery, and the faster it increases during storage and cycling. The internal resistance is related to the battery structure, battery material properties, and manufacturing process, and changes with ambient temperature and state of charge. Therefore, the development of low internal resistance batteries is the key to improving the power performance of batteries, and it is of great practical significance for battery life prediction to grasp the change law of battery internal resistance.

The meaning of resistance in lithium-ion batteries

With the use of lithium batteries, the battery performance continues to decay, mainly manifested as capacity decay, internal resistance increase, power decline and so on, which will be considerable important impact on the battery longer lifetime and safety performance. So here let’s talk about the topic of what’s the internal resistance.

Resistance is the resistance that current flows through the interior of the battery when the lithium battery is working. Generally, the internal resistance of lithium batteries is divided into ohmic internal resistance and polarization internal resistance. Ohmic internal resistance consists of electrode material, electrolyte, diaphragm resistance and contact resistance of various parts. Polarization internal resistance refers to the resistance caused by polarization during electrochemical reaction, including electrochemical polarization internal resistance and concentration polarization internal resistance. The ohmic internal resistance of the battery is determined by the total conductivity of the battery, and the polarization internal resistance of the battery is determined by the solid-phase diffusion coefficient of lithium ions in the electrode active material.

Ohmic resistance

Ohmic internal resistance is mainly divided into three parts, one is ionic impedance, the other is electronic impedance, and the third is contact impedance. We hope that the smaller the internal resistance of the lithium battery, the smaller the internal resistance, so we need to take specific measures to reduce the ohmic internal resistance for these three items.

1.Ion impedance

Lithium battery ion impedance refers to the resistance of lithium ions to transfer inside the battery. Lithium ion migration speed and electron conduction speed play an equally important role in lithium batteries, and ionic impedance is mainly affected by positive and negative electrode materials, separators, and electrolytes. To reduce the ionic impedance, the following points need to be done:

• ensure that the positive and negative materials and the electrolyte have good wettability—In the design of the pole piece, it is necessary to select an appropriate compaction density. If the compaction density is too large, the electrolyte will not easily infiltrate, which will increase the ionic impedance. For the negative pole piece, if the SEI film formed on the surface of the active material during the first charge and discharge is too thick, the ionic impedance will also be increased, and the formation process of the battery needs to be adjusted to solve this problem.

 

• The effect of electrolyte—The electrolyte should have suitable concentration, viscosity and conductivity. When the viscosity of the electrolyte is too high, it is not conducive to the infiltration between the electrolyte and the positive and negative active materials. At the same time, the electrolyte also needs a lower concentration, and if the concentration is too high, it is also not conducive to its flow and infiltration. The conductivity of the electrolyte is the most important factor affecting the ionic impedance, which determines the migration of ions.

 

• The effect of the diaphragm on ionic impedance—The main factors affecting the ionic impedance of the diaphragm are: electrolyte distribution in the diaphragm, diaphragm area, thickness, pore size, porosity and tortuosity coefficient. For ceramic diaphragms, it is also necessary to prevent ceramic particles from blocking the pores of the diaphragm, which is not conducive to the passage of ions. While ensuring that the electrolyte fully infiltrates the diaphragm, there must be no residual electrolyte remaining therein, which reduces the use efficiency of the electrolyte.

2.Electronic impedance

There are many influencing factors of electronic impedance, which can be improved from the aspects of materials and processes.

• Positive and negative plates—The main factors affecting the electronic impedance of the positive and negative plates are: the contact between the active material and the current collector, the factors of the active material itself, and the plate parameters. The active material should be fully in contact with the current collector surface, which can be considered from the current collector copper foil, aluminum foil substrate, and the adhesiveness of the positive and negative electrode paste. The porosity of the active material itself, the by-products on the surface of the particles, and the uneven mixing with the conductive agent will all cause changes in electronic impedance. Plate parameters such as the density of the active material are too small, the particle gap is large, which is not conducive to electron conduction.

 

• Diaphragm—The main factors affecting the electronic impedance of the diaphragm are: the thickness of the diaphragm, the porosity and the by-products during the charging and discharging process. The first two are easy to understand. After the dismantling of the battery cell, it is often found that a thick layer of brown material is attached to the diaphragm, including the graphite negative electrode and its reaction by-products, which will cause the diaphragm pores to block and reduce the battery life.

 

• Current collector substrate—The material, thickness, width and degree of contact of the current collector with the tabs all affect the electrical impedance. The current collector needs to choose an unoxidized and passivated substrate, otherwise it will affect the impedance. Poor welding between copper and aluminum foil and tabs will also affect electronic impedance.

3.Contact resistance

Contact resistance is formed between the contact between the copper and aluminum foil and the active material, and it is necessary to focus on the adhesiveness of the positive and negative electrode paste.

Polarization resistance

The phenomenon that the electrode potential deviates from the equilibrium electrode potential when a current passes through the electrode is called the polarization of the electrode. Polarization includes ohmic polarization, electrochemical polarization and concentration polarization. Polarization resistance refers to the internal resistance caused by the polarization of the positive and negative electrodes of the battery during the electrochemical reaction, which can reflect the internal consistency of the battery, but is not suitable for production due to the influence of operation and methods. The polarization internal resistance is not constant and changes with time during the charging and discharging process. This is because the composition of the active material, the concentration of the electrolyte and the temperature are constantly changing. Ohm’s internal resistance obeys Ohm’s law, and the polarization internal resistance increases with the current density, but it is not a linear relationship. Usually increases linearly with the logarithm of the current density.

Next time topic2 of The Factors affecting internal resistance of lithium ion batteries

 

The Frequency Regulation of MLTO

Applied in the Energy Storage Technology

Abstract: The energy storage system in the grid can enhance the power grid control and solve the high efficiency and comprehensive utilization of large scale intermittent renewable energy access.MLTO battery energy storage system with fast charging and discharging response, continuous 4C of the high rate charge discharge capacity, long service life, is very suitable for smart grid fast response energy storage applications. In this paper, the THINPACK  MLTO powder technology,container energy storage system design and 1000V ESS system applied in the North American market is introduced.

Key words : LTO technology; Energy storage; Quick respond; Frequency regulation:AFC

1.MLTO powder technology

LTO as an anode active material has excellent structural stability and fast charge-discharge capability in Li-ion battery applications. However, during the charging and discharging process, the gas generation problem caused by the side reactions of LTO batteries greatly affects its life and limits the large-scale application of LTO batteries.

Chemical side reactions that occur on the surface of LTO materials. During the charging and discharging process, the side reaction between the electrolyte and the LTO material  generates  a  reducing  lithium salt.During charging, the reducing lithium salt is oxidized and generates  by -products such as H₂O, CO,CO₂, C₂H4 and C3H6 . These side reactions allow the continuous generation of gas inside the battery and ultimately accelerate the decay of the battery. In order to solve the above problems, the research team of thinpackmodified the traditional LTO powder, and coated a layer of material with low chemical reactivity but good lithium ion conductivity on the surface of the powder to isolate the electrolyte and LTO . Contact between materials , thus avoiding the occurrence of side chemical reactions during the charging and discharging process of the battery.

Fig.1 Scanning electron microscope image of MLTO powder

 

Using MLTO powder as the negative electrode active material and the ternary material of shackles, diamonds and erbium as the positive electrode active material  has excellent cycle life and service life. At room temperature, 100% DOD cycling tests were carried out for two years under 6C charge and 6C discharge conditions , and the test results are shown in Figure 2 . It can be seen that after 25 000 deep charge-discharge cycles, the discharge capacity of the battery remains about 75 % of the initial capacity.

Fig. 2 Cycle life curve of MLTO battery

 

2.Containerized energy storage system design

The energy storage system uses a 23V/60AE standard battery module, as shown in Figure 3 . The cells are configured in 10 series and 6 parallels. The all-round protection bracket is designed to support and protect the cells, and the electrical connection between the cells is fully automatic laser welding to ensure the stability of the product. The reinforced heat dissipation fin design ensures the heat dissipation of the battery under high-rate cycling. The internal integrated LECU unit with automatic equalizationfunction is used for the measurement of the cell status in the module and external communication. Module mounting components, power connectors, communication connections, etc. are provided outside the module.

Fig.3 MLTO schematic diagram of battery

    Figure 4 shows a schematic diagram of the MLTO module installation. The module installation adopts a drawer structure, and the use of the guide rail makes the installation and replacement of the module very convenient. All electricity

The gas connection is designed as a blind-mate junction structure, all electrical connections to the module are completed after the module is inserted into the battery rack. The overall layout is clear and tidy. The system consists of two sets of battery cells. Each group of cells is 1000V/60AE, and the two groups of battery cells are connected in parallel. The maximum continuous charging and discharging power of the system reaches 500kW.

Fig.4 Schematic diagram of MLTO module installation

    The battery system, fire protection system, air conditioning system, monitoring system, control center, data server, etc. are integrated in the container to ensure the safety, stability and reliability of the battery system during operation. A schematic diagram of the interior of the container is shown in Figure 5 . The local server can upload the system operation data to the monitoring center for real-time monitoring through the Ethernet, and send the fault and alarm information that occurs during the operation to the monitoring center management personnel at the first time.

Fig. 5 Schematic diagram of the interior of the container

    Size of the container is a 20 -foot standard container, as shown in Figure 6 . And through the container transport identification and certification. The container system can be transported as an integral unit. All connections can be made in the junction box outside the container. The junction box is equipped with a glass window for easy observation of the status of the internal switchgear. Both the AC input circuit design and the DC output circuit design have manual disconnect devices to facilitate system power-off maintenance. At the same time, an indicator panel is also designed on the outside of the container to indicate the key design status of the whole system. Ensure real-time monitoring of key equipment during system operation .

Fig. 6 Schematic diagram of the exterior of the energy storage container

3.Application of MLTO battery energy storage system in North American fast frequency regulation market

PJM is the largest interconnected power system in North America with a total installed capacity of 106 000MW . In order to maintain a continuous balance of generation and load and maintain the system frequency, as shown in Figure 7 , regulation and frequency response services must be provided. In addition, the PJM simultaneously executes the load response plan, such as when the LMP of the system is high or an emergency occurs, the user can respond to reduce the load and can be rewarded. The L0TO battery energy storage system can well meet the working conditions of the frequency modulation market due to its excellent cycle life and rate capability.

Fig. 7 FM application intent

    Figure 8 shows the current response curve of the L/TO battery system under the PJMI condition . It can be seen from Figure 8 that the current response in frequency modulation applications is very frequent, and the power requirements of the energy storage system are very high, while the energy requirements are small. Therefore, the low-capacity and high-rate system design can meet the power requirements of the grid on the one hand, and on the other hand. On the one hand, the smaller battery capacity can effectively reduce the cost of the energy storage system.

Fig.8 Current response of MLTO energy storage system in

PJM frequency modulation

    Statistics also found that the on-time output rate of the battery under this working condition is very high, reaching 1800Ah/ day. Converted into battery cycle is about 10 times / day. The requirement of battery cycle ( converted to 100 % DOD, considering the life factor of different DOD cycles ) for one year of use has reached more than 2500 times. The L0TO with its 25 000 cycle life is ideal for this high cycle life requirement.

4   Conclusion

Based on advanced MLTO powder technology and battery system design and management ,THINPACK has developed a fast- response  battery energy storage system for the North American frequency modulation  market. The data shows thatthe application of the frequency modulation market has strict requirements on the power  characteristics and life characteristics of the energy storage system .THINPACK MLTO battery system just meets the strict requirements of the frequency regulation market for energy storage systems. In the FM market in the future, MLTO batteries will have broad space and excellent performance.

How graphene material improve the Li-ion performance

Organic compound materials are more and more widely used, and are also widely used in lithium batteries middle. The positive electrode material of lithium battery is generally metal oxide, and the negative electrode adopts organic compound material, which can greatly affect the performance of lithium battery. Carbon is one of the elements with the most compound forms in nature, and the hybrid forms of carbon atoms are various, which can form different carbon materials. The negative electrode material of lithium battery is generally dominated by graphite material, and graphene is a material composed of carbon atoms, and its performance is very outstanding. The interior of the graphene material is composed of multi-layer graphite single atomic layers, and each atomic layer forms a compound structure in the form of a hybrid orbital, that is, each carbon atom has an electron in the orbital that has not reached a saturated bond, so as to interact with the adjacent orbital. Unsaturated electrons combine. Graphene is not only a currently known material with high strength, but also the electron carrier in it conforms to the Hall effective, which can change the chemical potential energy through the action of an electric field. Therefore, graphene plays an important role in improving the performance of lithium batteries.

Fig 1. Schematic diagram of the structure of grapheme

                                                                     

Fig 2.  SEM of graphene

Application of Graphene in Anode Materials

   

The negative electrode of lithium-ion battery is mainly used as the main body of lithium storage, which plays a decisive role in the performance of the battery. Usually, the advantages and disadvantages of negative electrode materials are judged by judging whether they have good lithium ion transport channels and electron transport channels. Graphene has both the ability to provide good ability of good electron transport channels and excellent lithium ion transport properties. Graphene not only has excellent electrical conductivity, but also the interlayer spacing of graphene is extremely small, only in the order of micro and nanometers, which makes the diffusion path of lithium ions short; the combination of graphene and lithium ions is in the entire outer surface of graphene. Surface simultaneously, thus improving transfer performance.

Application of Graphene in cathode materials

The energy density of a lithium-ion battery is determined by the energy density of the positive electrode material, so the positive electrode material is a very critical part of each component unit of a lithium-ion battery. The current research shows that the composite cathode material formed by graphene and LiFePO₄, LiNi_1/3Co_1/3Mn_1/3O₂ and LiMn₂O₄ can improve the electrochemical performance of the material. Olivine-type LiFePO4 is a widely used cathode material for lithium-ion batteries with a theoretical specific capacity of 170mAh/g, but its application at high rates is limited due to low electronic conductivity and lithium ion diffusion. After compounding it with graphene, graphene fully wraps the surface of LiFePO4, and uses the tough network conductive structure in graphene to maintain the conductivity, thereby improving the conductivity of the entire material.

 

 

 

 

Thinpack Lithium titanate LTO battery is an improved battery of ordinary lithium titanate (LTO) battery. As the negative electrode material of lithium ion battery – lithium titanate, a 2.3V lithium ion secondary battery is formed with the positive electrode material – ternary material. Due to the high safety of lithium titanate (puncture, overcharge, short circuit, etc. will not catch fire), high stability, long life (can reach the same life as the vehicle), strong current charge and discharge capacity (charge and discharge current can reach 10C) It has the characteristics of green and environmental protection, and also has higher energy density and cost performance than super capacitors. It will be a better choice for hybrid electric vehicles and plug-in vehicle energy storage systems.

Thin pack LTO-14Ah lithium titanate battery is quite different from ordinary lithium batteries in terms of the chemical properties of the material itself. It is specially designed for fast-charging commercial vehicles, mining trucks, and heavy trucks (port tractors, urban construction and factory dump trucks) It is designed to meet the needs of other applications, with excellent performance such as fast charging, long life, and high safety. Zero-stress material, there is basically no volume change in the insertion and extraction of lithium ions. The material has super long-term stability, thus ensuring a super long cycle life; the nanostructure of the material ensures a higher tap density and can provide ultra-high rate charging and discharging capabilities (more than 16C charging and discharging); the material has a considerable Excellent electrochemical performance and safety characteristics, unparalleled super environmental endurance, and can maintain particularly excellent capacity performance under extreme temperature conditions.

Thin Pack LTO-14Ah® lithium titanate battery uses patented modified lithium titanate material as the negative electrode material, which effectively reduces the catalytic activity on the surface of the material, prevents the reaction between the electrolyte and the electrode, and suppresses the flatulence problem of the lithium titanate battery. , give full play to its long cycle life. At the same time, through the optimization of the electrolyte additive and solvent system, the infiltration of the electrolyte in the negative electrode is effectively improved, the gas production is reduced, and the cycle life of the cell is improved.

1. Ultra-fast charging – 10-minute ultra-fast charging, ultra-high rate discharge, taking into account both use efficiency and energy performance;

2. Ultra-long service life – up to ≥10,000 times of ultra-long cycle life (0~100% SOC full and full discharge), ideal return on investment period;

3. High safety – modified lithium titanate material, excellent fast charging performance, no lithium dendrite precipitation (minimizing the risk of internal short circuit and thermal runaway).

The LTO-14Ah lithium titanate battery has the advantages of ultra-fast charging, ultra-long service life, high safety and a wide operating temperature range, and is equipped with an industry-leading thermal management system and battery management system, bringing a new operation mode of new energy vehicles and operational efficiency.

The new operation mode completely solves the problem brought by the traditional slow-charging bus that “because the vehicle cannot be quickly recharged, the vehicle needs to run all day long to completely consume the battery, and it needs to be charged slowly for 6-8 hours overnight” at night. The operation mode brings: the large battery pack is expensive and inefficient; Single-pile slow charging of bicycles requires a large investment; daily deep discharge and deep charging have short battery life; additional investment such as battery replacement and other major operational problems have become a high-quality choice in the transportation field. Plug-in hybrid commercial vehicles, mining trucks, heavy trucks (port tractors, urban construction and factory dump trucks), etc.

There are following points
1. Longer life than ordinary lithium titanate (LTO) batteries
2. There will be no gas evolution of ordinary lithium titanate (LTO) batteries
3. Greater charge and discharge current than ordinary lithium titanate (LTO) batteries
4. Higher charging and discharging efficiency than ordinary lithium titanate (LTO) batteries
5. More stable than ordinary lithium titanate (LTO) batteries

 

A battery module refers to a battery cell combination module that meets the voltage and power requirements and is composed of cells in parallel and series, and then assembled with a battery management system to build a battery pack product.

The benefits of battery modules are mainly reflected in the following three aspects.

1. Better mechanical strength and protection

The battery module is equivalent to adding a protective shell to the cell, which can keep the cell in a stable state and play a role of fixed protection.

The battery module is usually composed of cells, plastic frames, pressure plates, wire harnesses, BMS slave plates, fasteners, etc. The plastic frame, insulating pads and pressure plates can play a good insulating effect.The pressure plates at both ends can not only make the cells tightly arrangement, there will also be mounting holes are connected with box fixedly , which has a good stabilization effect and improves the safety of the battery module.

2. Improve electrical performance

The battery module adopts laser welding, so the impedance of the lithium battery module is relatively small, the performance of the battery will be reduced less, and the heat dissipation performance of the module will be better, thereby improving the electrical performance.

3. Reduce after-sales maintenance costs and improve troubleshooting capabilities

There are several independent battery modules constituted a PACK system, in which BMS can monitor the condition of the cells in each module. If any one of the cells is short-circuited or faulty, the after-sales personnel can directly find the faulty module and cell. Efficiently handle faults and replace battery cells, which greatly reduces maintenance costs and is convenient and fast.

The standard module of Hongwei new energy battery adopts the soft-packed battery cell that has been independently developed and produced from material to structure for 16 years. It has better electrical performance and heat dissipation effect, and adopts automotive-grade BMS to provide one-stop battery solution.