Development of Lithium-ion Power Battery Positive Material

Review on the Development of Lithium-ion power battery Positive Material

1 Lithium manganate

LMO has the advantages of low raw material cost, simple synthesis process, good thermal stability, high magnification performance, and superior low temperature performance. In recent years, mainstream lithium battery companies in Japan and South Korea have used LMO as the preferred cathode material for large power batteries. The significant progress made by Japan and South Korea in the application of the manganese system positive pole, as well as the commercial application of the market representative models Nissan Leaf and General Volt, show the huge application potential of positive spinel LMO in the field of new energy vehicles.

1.1 Research Progress

The problem of poor high temperature cycle and storage performance of positive spinel LMO has always been the key to limit its application in dynamic lithium ion batteries. The poor high temperature performance of LMO 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 effective contact between particles, thus affecting Li+ diffusion and electrical conductivity between particles cause capacity loss.

(2) Oxygen defect: When the spinel is hypoxic, there will be simultaneous capacity attenuation on the 4.0 and 4.2 V platforms, and the more oxygen defects, the faster the battery capacity decay.

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

(4) The electrolyte decomposes at a high potential and forms a Li2CO3 film on the LMO surface, increasing the polarization of the battery, resulting in the attenuation of the capacity of the spinel LiMn2O4 during the cycle. Oxygen defects are a major cause of LMO's high temperature cycle decay, because LMO's high temperature cycle decay always increases with the reduction of Mn's valence.

How to reduce the Mn3 + in lithium manganate that causes the differentiation effect and increase the Mn4 + that is conducive to structural stability is almost the only way to improve the LMO high temperature defect. From this point of view, adding excess lithium or doping various modified elements is to achieve this goal. Specifically, improvements to LMO's 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) Surface control. The crystal morphology of LMO has a major influence on the dissolution of Mn. For spinel LMO, the dissolution of manganese occurs mainly on the(111) crystal surface. The ratio of lithium(111) manganese crystal surface can be reduced by controlling the spherical morphology of lithium mono-crystalline manganese acid, thereby reducing Mn dissolution.. Therefore, the high-end modified LMO with relatively good comprehensive performance is a single crystal particle.

(3) Table bread cover. Since the dissolution of Mn is one of the main reasons for the poor high temperature performance of LMO, the LMO table bread can be coated with an interface layer that can lead to Li + and isolate the electrolyte from the LMO, which can improve the high temperature storage and circulation performance of LMO[ 3] And ...

(4) Optimized composition of the electrolyte. The matching of the electrolytic liquid and battery processes to LMO performance is crucial. Since HF in the electrolyte is the culprit that causes Mn dissolution, it is the basic way to solve the high temperature performance of LMO by matching the positive electrode with the electrolyte, reducing the solubility of Mn, and thus reducing the destruction of the negative electrode.

(5) Mixed with binary / ternary materials. Since the energy density of high-end modified lithium manganate can increase in small space, LMO and NCA/NMC blending is a more realistic solution that can effectively solve the problem of low energy density of lithium manganate in separate use. For example, Nissan Leaf is a 11 % NCA mixed in LMO, and General Volt also adds 22 % of NMC and LMO mixed as positive materials.

1.2 Dynamic market analysis

The dissolution of manganese at high temperatures will be very serious for high-capacity lithium manganese. In general, LMO with a capacity higher than 100mA/g can not meet the demand for power at high temperatures. The power type LMO has a capacity of 95 to 100 mA/g, which determines that LMO can only be used on power-type lithium ion batteries. Therefore, for the current stage, electric tools, hybrid electric vehicles(HEV) and electric bicycles are the main applications of LMO.

From the price point of view, the current domestic high-end dynamic LMO price is generally 80,000 to 100,000 tons. If you consider that the Mn metal price is too low, LMO has basically no recycling value. Then LMO, like LFP, is a "one-time use" positive material. In contrast, NMC can make up 20 % to 30 % of raw material costs through battery recovery. Since LMO and LFP coincide in many application areas, LMO must reduce the price to low enough to have an overall value for money compared to LFP. Considering the reality that most LFP batteries occupy the domestic power cell market, high-end power LMO materials must reduce the price to about 60,000 tons before they can be accepted by the market on a large scale. Therefore, domestic lithium manganate manufacturers still have a long way to go.

Lithium iron 2 phosphate

As the first choice material for lithium-ion power batteries in China, lithium iron phosphate has the following advantages: First, the safety requirements of power cells are high, the safety performance of lithium iron phosphate is good, and no safety problems such as fire and smoke have occurred; Second, from the point of view of service life, lithium iron phosphate batteries can achieve a long life equivalent to the life cycle of vehicles; Third, in terms of charging speed, speed, efficiency and safety can be taken into account. Therefore, lithium-iron phosphate power battery is still the most suitable for the safety needs of domestic new energy passenger cars.

2.1 Research Progress

LFP has problems in energy density, consistency, and temperature adaptability. The most important defect in practical applications is batch stability. Regarding the consistency of LFP production, it is generally considered from the production stage, such as the lack of system engineering design for the construction of small trials to medium trials, medium trials to production lines, and the control of raw material state control and production process equipment state control issues. These are the reasons that affect the consistency of LFP production. However, the problem of LFP production consistency has fundamental thermodynamic reasons for its chemical reaction.

From the material preparation point of view, the synthesis reaction of LFP is a complex multi-phase reaction with solid phase phosphate, iron oxides, and lithium salts, plus carbon precursors and reducing gas phases. In this multiphase reaction, iron has the possibility of being reduced from +2 to elemental, and it is difficult to ensure the consistency of the reaction micro-region in such a complex multiphase reaction. The consequence is that trace amounts of +3 iron and elemental iron may exist 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 +3 iron can also be dissolved by the electrolyte and reduced at the negative electrode. From another perspective, LFP is a multi-phase solid-state reaction under a weak reducing atmosphere. It is inherently more difficult to control than the oxidation reaction to prepare other positive materials. The reaction micro-region will inevitably have incomplete reduction. The possibility of over-reduction, Therefore, the root cause of the poor consistency of LFP products lies in this.

The complete automation of the production process is currently the main means to improve the stability of LFP material batches. The difference between different batches of materials can only be increased to the acceptable range of LFP application through continuous improvement of the process and equipment. These include:

(1) Procurement of high-purity and high-specification raw materials, strengthening control from the source, and maximizing the purity and high stability of the product;

(2) Advanced automatic processing equipment is used in key production stages of key processes, and key parts of key equipment are continuously optimized to meet the requirements of material continuity and consistency;

(3) Strictly implement process discipline, strengthen process control, improve production efficiency, and ensure product inter-batch quality stability.

2.2 Dynamic Market Analysis

In view of the special nature of the large number of passengers, compared with small passenger vehicles such as cars, the importance of safety issues in the new energy passenger car industry should take precedence over performance issues such as mileage renewal. Therefore, the management of power battery systems should give primary consideration to safety factors. Comprehensively comparing the current mainstream battery technology route, it can be considered that lithium iron phosphate battery is the most suitable technology choice for electric passenger cars. At the same time, from the point of view of product technology, first, the power design of lithium iron phosphate batteries can also be quickly charged. The data after the use of Ningde era products by the leading Yutong passenger cars in the passenger car industry shows that: After using 80 % of the lithium iron phosphate batteries, they can be filled quickly and can safely reach 4,000 to 5,000 cycles; After 70 % use, fast charging can also guarantee 7,000 to 8,000 cycles. Secondly, at this stage, the 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 total. With the continuous expansion of the ternary material power battery, the LFP is changing. However, after the LFP power battery was introduced into 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 now, LFP batteries are still the mainstream of power cells for new energy vehicles. With the increasing demand of domestic power cell market, the mature LFP power market will also show a continuous positive growth trend.

3 Ternary material

3.1 Research Progress

The ternary material actually integrates the advantages of the three materials LiCoO2, LiNiO2, and LiMnO2. Due to the obvious synergistic effects between Ni, Co, and Mn, the performance of the NMC is superior to that of a single set of layered positive materials. The influence of the three elements in the material on the electrochemical properties of the material is also different. 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 cyclic performance[ 4] ; Mn can reduce costs and improve the structural stability and safety of materials[ 5] ; Ni as an active substance helps increase capacity. The ternary material has a higher specific capacity, so the energy density of the single core has a greater increase than that of LFP and LMO batteries.

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