Practical high voltage cathode materials for lithium battery

With the rapid development of fields such as electric vehicles and energy storage power stations, the development of lithium-ion batteries with high energy density has become a top priority in the field of new energy. One of the most effective strategies to increase the energy density of lithium ion battery is to increase the output voltage of the battery, which mainly depends on the high voltage cathode materials. At present, there are four main categories of positive electrode materials for high voltage lithium-ion batteries (> 4 V vs. Li/Li+), including lithium-rich layered oxides, nickel-rich layered oxides, spinel oxides, and high voltage polyanionic compounds. However, it is still facing severe challenges for these cathode materials to increase the output voltage while maintaining high capacity, excellent rate performance and long service life.

The failure mechanism of the positive electrode

The life of high voltage cathode materials is far less than that of traditional commercial LiFePO4 and LiCoO2 cathodes, which is mainly caused by the instability of the cathode itself and the cathode/electrolyte interface. Other materials inside the cathode, such as conductive carbon and binders, will also have a great impact on the performance of full cells based on high voltage cathode materials. The figure below shows the relationship between the electrochemical window of the electrode and the electrochemical window of the electrolyte, showing three mechanisms for the failure of the positive active material, including: oxygen evolution at the positive electrode, phase transition and fragmentation of positive electrode particles, and dissolution of transition metal ions.

Strategies to stabilize the cathode

Coating and doping

Coating and doping can greatly improve the structural stability of positive electrode active materials. For coating, both ceramic oxides and conductive polymers can promote the formation of stable CEI on the positive electrode surface. The surface of lithium cobalt oxide is coated with ionic conductor LAGP, so that the positive electrode of lithium cobalt oxide can cycle stably at a cut-off voltage of 4.6V. Doping of metal elements and non-metal elements in the high voltage positive electrode. Doping improves the structural stability of the secondary particles, inhibiting their fracture during cycling. The concentration gradient field strategy can adapt to the asynchronous volume strain of the inner and outer layers of the cathode, and further suppress the rupture of cathode particles.

Design of anti-oxidation electrolyte

The electrochemical window of conventional carbonate-based electrolytes (ECs) is less than 4.3 V, which cannot be matched with that of high voltage cathodes. Currently, several failure mechanisms for EC-based electrolytes have been proposed, including dehydrogenation reaction mechanism, nucleophilic reaction mechanism, and ring-opening reaction mechanism. It should be noted that these reaction mechanisms play a synergistic effect on the decomposition of EC-based electrolytes. The design of anti-oxidation electrolyte mainly focuses on the screening of anti-oxidation solvents and film-forming additives. In addition, high-salt electrolytes also have the potential to match high voltage cathode materials.

Modification of conductive carbon

The huge specific surface area and rich surface functional groups of conductive carbon make it unstable under high pressure, easy to oxidize and generate gas, and decompose. By grafting specific functional groups on the surface of conductive carbon and structural design of conductive carbon, the decomposition of conductive carbon under high pressure can be greatly suppressed.

New binder

The failure modes of commercial PVDF binders include: non-uniform coating, weak cohesion, and HF removal under alkaline conditions. To solve this problem, it can be solved by designing binders that form hydrogen bonds with the positive electrode, amorphous binders with high binding force, cation and H+ exchange binders, and free radical capture binders.

Cost control of high voltage cathode materials

The cost of the battery determines whether it can be accepted by the market. After years of development, the cost of lithium-ion battery pack has dropped from $600-900 per kWh in 2010 to less than $100 per kWh in 2021. Therefore, in order to survive in the highly competitive market, strictly controlling the cost of raw materials, manufacturing process, and final assembly is crucial for the development of high voltage Li-ion batteries. Although different battery systems have different cost ratios of different raw materials, the cost ratio of cathode materials is generally between 30% and 60% of the total raw materials. Especially in recent years, the rapid growth of demand for power batteries has led to a sharp rise in the cost of cathode materials. Therefore, how to reduce the cost of cathode materials has become a big problem in the battery industry. Especially for lithium cobaltate and ternary NCM cathode materials, due to the scarcity of cobalt resources, the price of cobalt remains high, which greatly affects the large-scale application of such cathode materials. Therefore, low-cobalt or even cobalt-free high voltage cathode materials will become a hot research topic in the future.

Safety assessment of high voltage cathode materials

Safety issues have always been one of the most concerned issues in the battery field. Therefore, it is very important to conduct a strict safety assessment of the battery life cycle from raw material production to use. The safety assessment of high voltage lithium-ion batteries includes two parts: one is production safety, and the other is battery operation safety. In the production process of batteries, the electrolyte solvents, additives, and electrode materials used should be as low-toxic and low-pollution as possible. At the same time, compared with traditional cathode materials such as lithium cobalt oxide and lithium iron phosphate, the thermal stability of high voltage cathode materials is worse, so more attention should be paid to the health status of high voltage lithium-ion batteries during operation.

Realistic evaluation of battery performance

The battery performance reported in most of the current research papers is based on the data obtained in the button battery, and the amount of electrolyte used in the button battery, the surface loading of the active material, and the battery capacity are all different from the actual conditions. The conditions of commercial batteries are far apart, which is not conducive to a proper evaluation of the performance of the designed electrode materials. Therefore, in the follow-up research work, it can be considered to evaluate the electrical performance in battery systems such as large batteries, pouch cells, and cylindrical batteries, so that the test results obtained will be closer to commercial requirements. Sustainable development of batteries

By the end of 2020, the number of electric vehicles in the world has exceeded 10 million, and it is expected to exceed 100 million by 2030. The service life of electric vehicle batteries is generally 5-8 years, so a large number of power batteries will be decommissioned in the near future. Therefore, the cascade utilization of batteries is an inherent requirement for the sustainable development of the battery industry and is of great significance to the development of a low-carbon economy. For the cascade utilization of batteries, the stability of the used batteries is worse and the safety hazards are greater. Therefore, it is necessary to establish a more complete battery health monitoring technology to monitor the cascade utilization of batteries in real time to ensure the safety of battery operation. At the same time, efficient recycling of old batteries is also the key to sustainable development of the battery industry.

Outlook

After years of development, researchers from various countries have made significant progress in the field of high voltage cathode materials, but there is still a long way to go before large-scale commercial applications. Only in this way can high voltage electrode materials with high performance, low cost, high safety, and sustainable development be successfully commercialized on a large scale.