All solid - state lithium batteries will be mass produced on the vehicle

Recently, academia and industry have given high expectations to all-solid-state lithium batteries. Solid-state battery companies are springing up at home and abroad. A number of world famous automobile enterprises have announced in 2017 that all solid state lithium batteries will be mass produced in vehicles from 2020 to 2025.

Many researchers and businesses think, relative to the lithium lithium sulfur, empty, aluminum and magnesium battery and graphene battery doesn't exist, all solid state lithium battery is the most potential alternative candidate of existing high energy density lithium ion battery technology, its energy density is expected to be 2 ~ 5 times that of the existing lithium ion battery, cycle and service life is longer, ratio, higher performance and can essentially solve the security problems of existing liquid electrolyte lithium ion battery.

If these goals are achieved, all-solid-state lithium batteries are bound to disrupt existing lithium-ion battery technology. In this paper, the technical difficulties and challenges of all-solid-state lithium battery are analyzed.

Short board of liquid electrolyte lithium ion battery

The volume energy density of cells for consumer electronics applications has reached 730W·h/L, and will develop towards 750 ~ 800W·h/L in the near future. The corresponding mass energy density is 250 ~ 300W·h/kg, and the circularity is 500 ~ 1000 times. The mass energy density of power battery has reached 240W·h/kg, and the volume energy density has reached 520 ~ 550W·h/L. In the near future, it will develop towards 600 ~ 700W·h/L, and the mass energy density will develop towards 300W·h/kg, and the circulation will reach more than 2000 times. The cycle life of energy storage battery has reached 7000 ~ 10000 times, and now it is developing towards 12000 ~ 15000 times. For the lithium ion battery with higher and higher energy density and liquid electrolyte, although a variety of improvement measures have been taken from the aspects of material, electrode, cell, module, power management, thermal management, system design and other aspects, the safety problem is still very prominent, and it is difficult to completely avoid heat runaway. In addition, the liquid electrolyte lithium ion battery cell also has the following main shortcoming.

(1) SEI film keeps growing. Due to the non-dense growth of SEI film and the large volume expansion and contraction of positive and negative electrode materials in the circulation process, some components of SEI film can be dissolved in the electrolyte, resulting in the continuous growth of SEI film on the positive and negative electrode surface, which leads to the decrease of active lithium, the continuous depletion of electrolyte, the continuous increase of internal resistance and pressure, and the expansion of electrode volume.

(2) dissolution of transition metal. For layered and spinel structure oxide anode material, the anode under the charging state in high oxidation state, prone to restore phase transition, the transition metal ions in skeleton after interaction with solvent in electrolyte precipitation to the electrolyte, and spread to the cathode, catalytic SEI further growth, at the same time, the anode material surface structure was destroyed, resistance increases, the irreversible capacity loss. Due to the role of transition metal catalyzing SEI film growth, the requirements for free magnetic metals of all materials in the battery reach below dozens of PPB levels, which also leads to the increase of the cost of battery materials.

(3) oxygen evolution from anode materials. For the high-capacity layered oxides, when charging to a higher voltage, the oxygen in the positive electrode lattice is prone to lose electrons, which are precipitated from the lattice in the form of free oxygen and oxidized with the electrolyte, leading to thermal runaway. The anode material structure is also gradually destroyed.

(4) electrolyte oxygenation. In order to improve the capacity of anode materials, it is necessary to charge to a high voltage in order to get rid of more lithium. At present, electrolyte solution for lithium cobalt oxide can be charged to 4.45v, and ternary materials can be charged to 4.35v. If the electrolyte continues to be charged to a higher voltage, the electrolyte will be oxidized and decomposed, and irreversible phase change will occur on the surface of anode.

(5) lithium evolution. Due to the slow internal dynamics of the materials embedded in the negative electrode, lithium metal precipitates directly on the surface of the negative electrode under low temperature overcharge or high current charge, which may lead to lithium dendrite and cause micro-short circuit. The highly active lithium metal reacts directly with the liquid electrolyte to reduce the active lithium and increase the internal resistance.

(6) high temperature failure. In the fully charged state, the negative electrode is in the reduced state, and the positive electrode is in the high oxidation state. At the same time, lithium salts also decompose spontaneously at high temperature and catalyze the electrolyte reaction. These reactions can lead to runaway heat. High temperatures can come from external sources as well as from internal short circuits, electrochemical and chemical exothermic reactions, high-current joule heat.

(7) volume expansion. After the use of a high-capacity silicon negative electrode, or after a high-temperature gas expansion, long-term cycle, due to the continuous decomposition of the electrolyte, SEI growth and reaction gas production, and the negative electrode itself volume expansion and contraction, the volume expansion of the soft-pack cell exceeds the application requirements within 10%.

If the all-solid-state battery cell can be successfully developed, its high-temperature safety and thermal runaway behavior may be improved, thereby simplifying or eliminating the cooling system and optimizing the thermal management system. The internal serial design can also be adopted to further save the weight of the collector fluid. Compared with the liquid electrolyte cell with the same energy density, the energy density of the system will be higher, and the decline ratio of the energy density of the all-solid electrolyte cell to the system should be lower. Therefore, from the perspective of battery system, the energy density of all-solid battery system may be slightly higher than that of liquid electrolyte battery system for the system with the same positive and negative materials.

One of the most important drivers for the development of all-solid-state lithium batteries is safety. Battery safety is of Paramount importance for all applications. The core problem of battery safety is to prevent thermal runaway and thermal diffusion. The condition of heat runaway is that the rate of heat generation is higher than the rate of heat dissipation. Therefore, if the cell can work at high temperature, or the initial temperature of thermal runaway is significantly higher than the normal operating temperature of the cell, the safety of the cell should be greatly improved in terms of overheating, large current, and internal short circuit. For the safety requirements of needling and extrusion, the charging and discharging depth (SOC) of the cell is required, and no violent oxidation reaction or other exothermic chemical and electrochemical reaction will occur due to internal short circuit and encounter with oxygen, water and nitrogen in the air during the whole life cycle.

The chemical and electrochemical stability of sulfides and polymers needs to be further improved. In fact, compared with liquid electrolyte cells, there has not been a report that the comprehensive electrochemical performance of solid electrolyte all-solid lithium battery cells is higher than that of liquid electrolyte cells. At present, the research focus is still on solving the characteristics of circularity and multiple, and there are very few test data on the thermal runaway and thermal diffusion behavior of all kinds of all-solid lithium batteries. Solidstatebatter * and [(safety) or (thermalrunaway)] were used as keywords to search the core collection of WebofScience, and 138 literature results were obtained in 2017.

After screening, only 9 papers mentioned the safety of solid state batteries, but most of the safety tests were to burn the electrolyte with flame or to study the microstructure change of materials under heating conditions or to strengthen the interface between lithium metal and solid state electrolyte, without conducting overall safety tests on solid state batteries. ZAGHIB and polymer electrolyte is analyzed and compared with the heating rate of liquid electrolyte thermal runaway, Japan's Toyota lanthanum doped lithium niobate was studied by DSC, academia sinica (LLZNO) zirconium oxygen generate heat behavior of all solid state lithium ion battery, finally it is concluded that all solid state lithium ion battery can improve security (reduced to liquid heat yield 30%) but not absolutely safe. Obviously, whether all-solid lithium ion battery really solves the intrinsic safety of lithium ion battery still needs more extensive and in-depth research and data accumulation.

Is concluded that in the whole life cycle of all solid state lithium ion batteries and solid-state metal lithium battery safety will be significantly better than that of optimized liquid electrolyte of lithium ion batteries too early, and based on the different solid electrolyte solid-state lithium battery may also have significant differences in security, require the system to study. If solid-state batteries and high temperature cycle characteristics of high temperature thermal runaway is superior to liquid electrolyte batteries, are in module and system level, through the power management, thermal management system, can further prevent batteries thermal runaway and thermal diffusion, relative to the liquid electrolyte batteries, protective insulation materials can better application in the module and system, not like this, both the cooling and heat insulation.

Dynamics, liquid electrolyte of lithium ion battery electrode electrochemical reaction area is the actual geometry size of tens to hundreds of liquid electrolyte of high ionic conductivity, relatively low contact resistance, make the lithium ion battery internal resistance of batteries in 10 ~ 15 m Ω/a. h, such work in high current, low batteries fever. The internal resistance of the cell mainly includes negative electrode, solid electrolyte membrane and positive electrode. Improving the ionic conductivity and reducing the film thickness are effective ways to reduce the surface resistance of each part. At present, all the parts of the solid-state lithium batteries at room temperature surface resistance cannot be reduced to the level of 10 m Ω/cm2.

The internal resistance is too high, leading to the heating of the cell when it is about to be charged, which is unacceptable for the application field without cooling system, but the working temperature requirement is not too high, such as mobile phones, tablet computers and other consumer electronics. The most challenging aspect of all-solid-state electrolyte cells is that during the charging and discharging process of positive and negative electrodes, the particles expand and contract in volume, and the physical contact between the solid electrolyte phase and the particles of positive and negative active substances may become worse. If lithium metal or composite materials containing lithium metal are used for the negative electrode, another big challenge is that in the case of high current density, lithium metal is preferentially precipitated at the interface, and if the precipitated lithium occupies the interface, the electrochemical reaction area will be gradually reduced. The development kinetics is excellent. In the case of full SOC, the material and electrode design of lithium deposition sites in the electrode instead of the main interface is the focus and difficulty of future research. From the current research progress, the development of all-solid-state lithium battery still needs a variety of comprehensive solutions to improve the dynamic characteristics of each part.

The results show that the energy density of all solid state batteries is lower than that of liquid electrolyte cells. Only when lithium metal is used as the negative electrode in the cell can the energy density of the cell be significantly higher than that of graphite or silicon. At present, the energy density of the lithium ion battery cell has reached the level of 300W·h/kg, 730W·h/L. If the energy density is higher than 2 times, the energy density of the battery cell needs to reach 600W·h/kg and 1460W·h/L. Although this is possible, it is far beyond the level of the existing technology, let alone 5 times. In addition, it is of no practical significance to simply emphasize the energy density of the cell. The actual application needs to meet more than 8 ~ 20 technical parameters at the same time, so it is of more practical significance to discuss the energy density of the cell under this premise. Even though the energy density of lithium metal battery can be significantly higher than that of lithium ion battery according to calculation, the recycling, safety and multiplier characteristics of lithium metal anode are far from meeting the application requirements.

For power and energy storage applications of large-capacity all-solid lithium batteries (more than 10A·h), no enterprise has reported the electrochemical data and safety data of the system at present. There are few studies on thermal runaway and thermal diffusion behavior, let alone the safety behavior of the whole life cycle. On the electrochemical performance and security advantages have not been research and validation is clear, and can be mass production of the material system, the electrodes and the electrolyte membrane materials, the design of batteries and intelligent manufacturing equipment is not yet mature, the corresponding BMS, thermal management system has not yet developed, the battery under the condition of cost accounting has not been clear, propaganda solid-state lithium battery can realize commercialization in the short term, especially use directly on the electric car is probably more dream than reality. Even in Japan, there are different opinions on whether and when all-solid-state lithium batteries with sulfide electrolytes can finally be applied. The challenges brought by air sensitivity, easy oxidation, high interface resistance and high cost are not easy to be completely solved in a short time, and it still needs continuous efforts.

According to the result of calculation, as a result of the anode materials with lithium batteries has more room to improve the energy density, lasts from solve metal lithium and electrolyte side reaction and increasing the safety of the lithium metal anode, solid-state lithium batteries should have an advantage, is the future of battery technology needs to be further studied, is worth having dream, need to work hard as soon as possible in order to find the excellent comprehensive performance, security, and at the same time the price will foot the equilibrium solution of application requirements.

As is expected to achieve faster transition technology, mixed solid liquid electrolyte containing a small amount of liquid electrolyte of lithium ion battery, li-ion solid composite metal lithium battery, may be on the basis of the existing liquid electrolyte of lithium ion battery, gradually improve security, energy density, and maintain a high rate, low resistance and low cost characteristics, therefore is expected to quickly enter the market, of course, mixed solid liquid electrolyte of lithium battery is faced with many technical challenges, need to overcome them one by one. Whether it is mixed solid-liquid electrolyte battery or all-solid battery, whether it is lithium ion or lithium metal, it will eventually win the market. To surpass the lithium ion battery technology which is still developing, it needs to pass solid basic research and unremitting efforts as well as goal-oriented and effective innovative solutions.

Verification technique can successfully, obviously can't rely on the new concept is put forward, published in the top academic journal articles, a large number of references and patent application and authorization, cannot only see a single technical indicators of progress, but need from all kinds of customers and strict, standard and system of the third party test data and the practical application.

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