Research progress on cathode materials for lithium ion power batteries

With the strong support of governments, new energy vehicle technologies are receiving more and more attention and rapid development. As the core technology of electric vehicles, research on power batteries has become the key. Lithium-ion batteries are recognized as the most promising power battery for electric vehicles due to their high specific capacity, long cycle life, low self-discharge rate, no memory effect, and environmental friendliness.

As the core material of the four materials of lithium ion power battery, the positive electrode material plays a vital role in the final performance of the battery. The performance optimization of the power battery often relies on the technological breakthrough of the positive electrode material, so the research of the positive electrode material becomes the current lithium ion. The most concerned sector of the power battery. Currently, commercial lithium ion battery positive materials mainly include lithium manganate (LMO), lithium iron phosphate (LFP), and ternary materials (NMC). The basic performance of the three materials is shown in Table 1. This paper discusses these three materials separately from the research progress and market application.

1 lithium manganate

LMO has the advantages of low raw material cost, simple synthesis process, good thermal stability, excellent rate performance and low temperature performance. The mainstream lithium battery companies in Japan and South Korea have adopted LMO as the preferred cathode material for large power batteries in recent years. The significant progress made by Japan and South Korea in the application of manganese cathodes and the commercial application of the market-leading models Nissan Leaf and General Volt show the great potential of positive spinel LMOs in the field of new energy vehicles.

1.1 Research progress

The problem of poor high temperature cycling and storage performance of the spinel LMO has been the key to limiting its application in power lithium-ion batteries. The poor performance of LMO at high temperatures is mainly caused by the following reasons:

(1) Jahn-Teller effect [1] and formation of passivation layer: the crystal system due to surface distortion is incompatible with the cubic crystal system inside the particle, which destroys the structural integrity and the effective contact between the particles, thus affecting Li+ diffusion and electrical conductivity between particles cause capacity loss.

(2) Oxygen Defect: When the spinel is deficient in oxygen, capacity decay occurs simultaneously on the 4.0 and 4.2 V platforms, and the more oxygen defects, the faster the battery capacity decays.

(3) Dissolution of Mn: Traces of water present in the electrolyte react with LiPF6 in the electrolyte to form HF, causing disproportionation of LiMn2O4, dissolution of Mn2+ into the electrolyte, and destruction of the spinel structure, resulting in LMO cells Capacity attenuation.

(4) The electrolyte is decomposed at a high potential, and a Li2CO3 film is formed on the surface of the LMO to increase the polarization of the battery, thereby causing the capacity of the spinel LiMn2O4 to decay during the cycle. Oxygen deficiency is a major cause of LMO high temperature cycle decay because LMO high temperature cycle decay is always accompanied by a decrease in the valence of Mn.

How to reduce Mn3+ which causes disproportionation in lithium manganate and increase Mn4+ which is favorable for structural stability is almost the only way to improve the high temperature defects of LMO. From this point of view, the addition of excess lithium or doping various modifying elements is to achieve this. Specifically, improvements to LMO high temperature performance include:

(1) Heteroatom doping, including cation doping and anion doping. The cation doping elements that have been studied include Li, Mg, Al, TI, Cr, Ni, Co, etc. The experimental results show that the doping of these metal ions more or less will improve the cycle performance of LMO, the most obvious effect. It is doped with Al[2].

(2) Morphology control. The crystal morphology of LMO has a major impact on the dissolution of Mn. For spinel LMO, the dissolution of manganese mainly occurs on the (111) crystal plane, and the ratio of lithium manganate (111) crystal plane can be reduced by controlling the spheroidization of the micromorphology of lithium single crystal manganate. Reduce the dissolution of Mn. Therefore, the high-end modified LMOs with good comprehensive performance are all single crystal particles.

(3) Surface coating. Since the dissolution of Mn is one of the main reasons for the poor high temperature performance of LMO, the high temperature storage and cycle performance of LMO can be improved by coating the surface of LMO with an interface layer capable of conducting Li+ and isolating the contact of electrolyte with LMO. [3].

(4) Electrolyte optimized components. The matching of the electrolyte and battery process is critical to the performance of the LMO. Since HF in the electrolyte is the main culprit for Mn dissolution, it is the basic way to solve the high temperature performance of LMO by matching the positive electrode and the electrolyte to reduce the solubility of Mn and thus reducing the damage to the negative electrode.

(5) Blending with binary/ternary materials. Because the energy density of high-end modified lithium manganate can be increased, LMO and NCA/NMC blending is a realistic solution to effectively solve the energy density deviation of lithium manganate in single use. Low problem. For example, the Nissan Leaf blends 11% of the NCA in the LMO, and the Universal Volt also incorporates 22% of the NMC and LMO blends as the cathode material.

1.2 Power Market Analysis

The excessively high capacity of lithium manganate dissolves manganese at high temperatures. In general, LMOs with a capacity higher than 100 mA/g cannot meet the power requirements. The power LMO has a capacity of 95~100 mA/g, which determines that LMO can only be used on power lithium-ion batteries. Therefore, at this stage, power tools, hybrid electric vehicles (HEVs) and electric bicycles are the main application areas of LMO.

From the price point of view, the current price of high-end power LMOs in China is generally between 80,000 and 100,000/ton. If the price of Mn metal is too low, the LMO has no value for recycling. LMO is the same as LFP. Sexual use of the cathode material. In comparison, NMC can make up 20% to 30% of raw material costs through battery recycling. Since LMO and LFP are coincident in many applications, LMO must lower the price sufficiently to be more cost effective than LFP. Considering the current situation of most LFP batteries in the domestic power battery market, high-end power LMO materials must reduce the price to about 60,000 / ton, and there will be a large-scale acceptance of the market, so domestic manganese Lithium acid manufacturers still have a long way to go.

2 lithium iron phosphate

As the preferred material for domestic lithium-ion power batteries, lithium iron phosphate has the following advantages: First, the safety requirements of power batteries are high, and the safety performance of lithium iron phosphate is good, and there are no safety problems such as fire and smoke; second, from From the perspective of service life, lithium iron phosphate battery can achieve a long life equivalent to the life cycle of the vehicle; third, in terms of charging speed, speed, efficiency and safety can be considered. Therefore, the lithium iron phosphate battery is still the most suitable for the safety requirements of domestic new energy buses.

2.1 Research progress

LFP has problems in energy density, consistency and temperature adaptability. The most important defect in practical applications is the batch stability problem. Regarding the consistency of LFP production, it is generally considered from the production process, such as small test to pilot test, lack of system engineering design from the pilot test to the production line construction process, as well as raw material state control and production process equipment state control issues, etc. The reasons that affect the consistency of LFP production. However, the LFP production consistency problem has its fundamental cause of chemical reaction thermodynamics.

From the perspective of material preparation, the synthesis of LFP is a complex heterogeneous reaction with solid phase phosphates, iron oxides and lithium salts, plus carbon precursors and a reducing gas phase. In this heterogeneous reaction, iron has the potential to be reduced from the +2 valence to the elemental mass, and it is difficult to ensure the consistency of the reaction microdomains in such a complex multiphase reaction. The consequence is a trace of +3 valence iron. And elemental iron may be present in the LFP product at the same time. Elemental iron causes a micro short circuit in the battery, which is the most taboo substance in the battery, and the +3 valent iron can also be dissolved by the electrolyte and reduced at the negative electrode. From another point of view, LFP is a multi-phase solid state reaction under a weak reducing atmosphere. It is inherently difficult to control the oxidation reaction of other positive electrode materials, and the reaction microdomains will inevitably have incomplete reduction and excessive reduction. The possibility, so the root cause of the poor consistency of LFP products is here.

The full automation of the production process is currently the primary means of improving the stability of LFP material batches. Differences between different batches of material can only be increased to within acceptable fluctuations of LFP practical applications through continuous improvement of processes and equipment. Specifically include:

(1) Purchasing high-purity and high-standard raw materials, strengthen control from the source, and ensure product purity and high stability to the greatest extent;

(2) The key production processes of key processes are all advanced automatic processing equipment, and the key parts of key equipment are continuously optimized and optimized to meet the continuous and consistent production requirements of materials;

(3) Strictly implement process discipline, strengthen process control, improve production efficiency, and ensure product quality stability between batches.

2.2 Power Market Analysis

In view of the special nature of passengers, compared with small passenger cars such as cars, the importance of safety issues in the new energy bus industry is prioritized over performance issues such as driving mileage. Therefore, power battery system management should consider safety factors first. Comprehensive comparison of current mainstream battery technology routes, it can be considered that lithium iron phosphate battery is currently the most suitable technology choice for electric buses. At the same time, from the perspective of product technology, first of all, the lithium iron phosphate battery designed by power can also be quickly charged. The bus industry leader Yutong bus uses Ningde era products after the data shows: lithium iron phosphate battery used 80% after fast charge, can safely reach 4 000 ~ 5000 cycles; use 70% after fast charge, can also guarantee 7 000 ~8 000 cycles. Secondly, at this stage, the mass production maturity of lithium iron phosphate is higher than that of ternary materials and multi-component composites; from the material level, lithium iron phosphate has higher safety than ternary materials and multi-component composites.

In the Chinese power battery market, LFP batteries account for about 80% of the market. With the continuous expansion of ternary material power batteries, the situation of LFP's unique show is changing. However, after the LFP power battery was introduced to China, from the new energy vehicles at the 2010 Shanghai World Expo to the tens of thousands of pure electric vehicles in the domestic market, LFP batteries are still the mainstream of power batteries for new energy vehicles. As the domestic power battery market continues to increase, the increasingly mature LFP power market will also show a positive positive growth trend.

3 ternary materials

3.1 Research progress

The ternary material actually combines the advantages of LiCoO2, LiNiO2 and LiMnO2. Because of the obvious synergistic effect between Ni, Co and Mn, the performance of NMC is better than that of single-component layered cathode material. The three elements in the material have different effects on the electrochemical properties of the material. Co can effectively stabilize the layered structure of the ternary material and inhibit the cation mixing, improve the electronic conductivity of the material and improve the cycle performance [4]; Mn can reduce the cost. Improve the structural stability and safety of the material [5]; Ni as an active substance helps to increase the capacity. The ternary material has a high specific capacity, so the energy density of the single cell is greatly improved compared to the LFP and LMO cells.

In recent years, the research and industrialization of ternary material power batteries have made great progress in Japan and South Korea. It is widely believed that NMC power batteries will become the mainstream choice for electric vehicles in the future. In general, based on safety and recycling considerations, ternary power batteries mainly use 333, 442 and 532, which are relatively low in Ni content, but PHEV/EV requires higher energy density. 622 is also receiving more and more attention in Japan and South Korea.

The main problems with the current NMC application to power batteries are:

(1) Safety: The safety of ternary material batteries is more serious, leading to more serious safety problems;

(2) Cycling: The material is damaged during the repeated charging and discharging process, resulting in poor material circulation;

(3) Energy density: the secondary spherical particles formed by the granule agglomeration of the ternary material, because the secondary particles will be broken under higher compaction, thereby limiting the compaction of the ternary material electrode, thereby limiting the battery core. Further increase in energy density.

3.1.1 Security issues

Compared with LFP and LMO batteries, NMC batteries have outstanding safety problems. They are mainly difficult to pass through overcharge and acupuncture conditions, battery airflat is more serious, and high temperature cycle is not ideal. The safety of the ternary cell needs to be carried out in both the material itself and the electrolyte in order to receive the desired effect. The modification is mainly carried out from the following aspects:

(1) From the NMC material itself, it is first necessary to strictly control the surface residual alkali content of the ternary material. Alumina coating is the most common and the effect is obvious. Alumina can be either liquid-phase coated in the precursor stage or solid-phase coated in the sintering stage, as long as the method is effective.

(2) Secondly, the structural stability of NMC is improved, mainly by doping with hetero atoms. At present, the anionic and cationic composite doping is more used, which is beneficial to improve the structure and thermal stability of the material.

(3) The safety of the ternary cell also needs to be combined with the improvement of the electrolyte, which requires the battery manufacturer and the electrolyte manufacturer to jointly research the electrolyte formula suitable for the ternary material.

3.1.2 Cyclical issues

One of the most basic requirements for power batteries is long cycle life. Currently, it is required to match at least half of the life of the vehicle (8 to 10 years), and the 100% depth of discharge (DOD) cycle should be more than 5,000 times. At present, the cycle life of ternary materials can not achieve this goal. The best cycle record of ternary materials reported in the world is the ternary cell of NMC532 made by Samsung SDI. The cycle life of 0.5 C at room temperature is close. 3 000 times. The main modification measures for ternary materials are shown in Table 2.

3.1.3 Energy density problem

(1) Increase the Ni content. For NMC, the specific capacity increases with increasing Ni content, so increasing the Ni content in the material helps to increase the energy density. At the same time, however, the negative effects caused by increasing the nickel content are also very obvious. Because the Ni content in the Li layer is more obvious as the nickel content increases [6], it will directly deteriorate its cycle and rate performance. Moreover, the increase of nickel content makes the crystal structure stability worse, and the surface residual alkali content also increases. These factors will lead to safety problems, especially in the high temperature test conditions, the battery gas production is very serious. Therefore, the ternary material is not the higher the nickel content, the better, we must comprehensively weigh the indicators requirements of all aspects.

(2) Increase the compaction density. At present, the ternary materials commonly used in the market are mostly secondary spherical particles formed by submicron primary grain agglomeration, and there are many gaps between primary grains. This microscopic particle morphology results in a low compaction density of the ternary material, which limits the further increase in the energy density of the ternary material. Micro-scale primary single crystal particles similar to lithium cobaltate can be synthesized by using a novel precursor preparation process and three-dimensional free sintering technology [7]. The prepared ternary material of the micron-sized primary single crystal particles has a more complete crystal structure and a higher compaction density.

3.2 Power Market Analysis

Compared with LMO and LFP, ternary materials are more suitable for power tools and power batteries. In recent years, the demand for power battery energy density of electric vehicles has increased significantly. Automobile manufacturers have begun to test ternary batteries on HEVs and plug-in hybrid electric vehicles (PHEVs). If the energy density requirements of HEV are lower than the requirements of energy density, LMO, LFP and NMC batteries can meet the requirements; PHEV has higher energy density requirements, and only NMC/NCA batteries can meet PHEV requirements. Under the influence of Tesla's power battery technology route, NMC will inevitably have a tendency to expand applications in pure electric vehicles (EVs).

Japan and South Korea have shifted the focus of power battery research and development from LMO batteries to NMC batteries. This trend is very obvious. The three hard indicators issued by the Ministry of Industry and Information Technology for new energy vehicle power battery companies are: in 2015, the specific energy of single cells reached 180 Wh/kg or more (modular specific energy 150 Wh/kg or more), the cycle life exceeded 2,000 times or the calendar life reached In 10 years, the cost is less than 2 yuan / Wh. At present, only NMC batteries can meet these three indicators at the same time. Therefore, NMC will become the mainstream cathode material for power batteries in the future, and LMO and LFP will only be able to compete in the supporting role due to their own shortcomings. There are A>C>B>D. When the grid uses this package to serve users, the user can have four options in Table 1. In the actual situation, the access of the fan can only be determined by the grid, so the user can only choose one of the cases 1 and 3, or choose one of the cases 2 and 4.

3.2.1 Power supply reliability

Considering the price of electricity from the perspective of power supply reliability, the higher the reliability, the higher the price of electricity. Therefore, the price of the non-interruptible area is greater than the price of the area that can be interrupted (the unreliable power supply reliability is higher than the interruptible power supply reliability), and the price of the connected fan is greater than the price of the non-connected fan (here) For a single-time discussion, regardless of the intermittent nature of the fan, only the fan access is added to the capacity considerations). This leads to: D>A>C>B. When the grid uses this package to serve users, the user can have four options in Table 1. In the actual situation, the access of the fan can only be determined by the grid, so the user can only choose one of the cases 1 and 3, or choose one of the cases 2 and 4.

3.2.2 Comprehensive consideration of power generation costs and power supply reliability

The weighting factor is used when considering the power generation cost and the reliability of the power supply. The electricity price in terms of power generation cost is h1, the electricity price in terms of power supply reliability is h2, and the electricity price in consideration of comprehensive consideration is h3, then: where: l1 and l2 are weighting coefficients, and the range is (0, 1). The grid can determine the size of the two weighting coefficients according to the actual situation, and thus set the size of A, B, C, and D. When the grid uses this package to serve users, the user can have four options in Table 1. In the actual situation, the access of the fan can only be determined by the grid, so the user can only choose one of the cases 1 and 3, or choose one of the cases 2 and 4.

4 Conclusion

At present, cathode materials for commercial lithium-ion power batteries mainly include lithium manganate (LMO), lithium iron phosphate (LFP), and ternary materials (NMC). Each material has its own advantages and disadvantages, and has its own application fields and market demands. Among them, power tools, HEVs and electric bicycles are the main application areas of LMO. New energy public transportation buses and taxis will still be dominated by LFP, and NMC power batteries will become the mainstream of future development, and will be high-end in the next 3 to 5 years. The ternary system of power lithium batteries will be in short supply. In the short-term, domestic lithium-ion batteries will still be based on lithium iron phosphate and lithium manganate. Domestic lithium batteries and electric vehicle companies can form mature batteries in 2 to 3 years through mastery of lithium iron phosphate materials. Technology, further improve the technical level, and then transition to the technical route of ternary materials. Therefore, materials and battery manufacturers have stepped up their layout in ternary materials, which has become a more urgent strategic issue.

5 Conclusion

With the sharp increase in the proportion of clean energy generation, safety, economy and environmental protection have become the focus of power development. Through the above analysis, it can be seen that the access of clean energy is more effective in reducing the total cost of social input, while the interruptible load mechanism in the demand side response has significant benefits in mitigating congestion, and the introduction of clean energy into the demand side response management, both Complement each other to work together in the system, the total social benefit is greater than the individual action, and the congestion cost is lower; after the clean energy is connected, some of the originally interrupted load resumes operation, which makes the system have more spare capacity and thus enhance The operability of the demand side response. At the same time, the time during which the interrupted load can be cut off is reduced, so that the user's electricity consumption is more stable and reliable, and both the supply and the demand benefit from both.

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