For lithium-ion batteries to be widely adopted, manufacturing costs are relatively high due to factors such as online maintenance and recycling, battery lifespan, system safety, and the sustainable development of the entire industry. Overcoming these challenges requires developing new battery technologies that combine low cost, long lifespan, high safety, and easy recyclability.
Lithium-ion batteries have become a highly sought-after choice for power batteries in new energy vehicles due to their advantages such as high energy density, low self-discharge rate, and high cycle efficiency. Data shows that in the first half of 2018, the global installed capacity of newly commissioned electrochemical energy storage projects reached 697.1 MW, a year-on-year increase of 133% and a 24% increase compared to the end of 2017. In terms of technology distribution, lithium-ion batteries had the largest installed capacity at 690.2 MW, accounting for 99% of the total, a year-on-year increase of 142%. In my country, the newly commissioned installed capacity of electrochemical energy storage projects reached 100.4 MW, a year-on-year increase of 127%, with lithium-ion batteries having the largest installed capacity at 94.1 MW.
"Replacing traditional diesel generators with lithium-ion batteries has a wide range of applications in special exercises, hospital rescue, communications, and emergency traction, offering more flexible and convenient applications," said Dong Shanmu, associate researcher at the Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, at the recent Second Symposium on the Development Direction of Energy Storage Battery Technology, outlining a "blueprint for lithium's ideal applications beyond electric vehicles." Liu Yong, secretary-general of the Energy Storage Application Branch of the China Chemical and Physical Power Sources Industry Association, stated even more directly that with the rapid development of electric vehicles, the development potential of lithium-ion batteries is enormous in the next 3-5 years.
The prospect of lithium batteries sounds incredibly promising, but as the lithium battery industry develops, some prominent problems are emerging, and the reality is becoming increasingly stark. While optimistic about the application prospects of lithium batteries, Dong Shanmu also expressed concerns about their safety.
Dr. Liu Hao from the Energy Storage Technology Research Group at the Institute of Electrical Engineering, Chinese Academy of Sciences, stated that from portable electronic devices to electric vehicles and then to large-scale energy storage power stations, the market's requirements for lithium-ion battery performance are gradually shifting from high energy density to low cost and high safety.
Currently, large battery technology on the market is still developed from small battery technology, and its manufacturing cost is relatively high. Because online maintenance and recycling issues have not been considered, the cost of the entire industry chain is relatively high, the battery life is also limited, and system safety issues are associated with it, making the development of the entire industry unsustainable.
How to solve these challenging problems? "We should develop new battery technologies that combine low cost, long lifespan, high safety, and easy recyclability," said Liu Hao.
To achieve a balance between rigidity and flexibility in electrolytes.
The most significant safety hazard of lithium batteries comes from the electrolyte, as the currently used liquid organic electrolyte is flammable and explosive. Dong Shanmu stated that replacing the liquid electrolyte with a solid electrolyte is widely recognized in the industry as the most effective way to improve the safety performance of lithium batteries.
"Currently, the solid electrolytes used in solid-state batteries generally have performance shortcomings and are still far from meeting the requirements of high-performance lithium-ion battery systems. In addition, the failure behavior of the 'solid-solid contact interface' and the underlying failure mechanism also need to be elucidated." Dong Shanmu believes that building high-performance solid-state batteries requires a two-pronged approach: constructing high-performance solid electrolytes and improving the compatibility and stability of the interface.
Dong Shanmu introduced a "rigid-flexible" design concept, where "rigid" refers to the rigid polymer framework and rigid inorganic particles, while "flexible" refers to the flexible polymer ion transport material. Through Lewis acid-base interactions between polymers and between polymers and inorganic particles, new channels for lithium-ion transport can be created, significantly improving the overall performance of the electrolyte. Furthermore, his team has developed a series of lithium salts that are compatible with polymer electrolytes, increasing the ion transference number of the electrolyte and thus improving the ion transport performance of solid-state electrolytes.
Currently, the solid-state battery designed by Dong Shanmu's team has an energy density of up to 291.6 watt-hours per kilogram, a capacity retention rate of over 85% after 850 cycles, and can pass five nail penetration tests without catching fire or exploding. In addition, the voltage can recover quickly after a short period of time.
"Based on the aforementioned single-cell technology, we collaborated with the Institute of Deep-Sea Science and Engineering of the Chinese Academy of Sciences to successfully demonstrate the application of the 'Qingneng-1' solid-state battery system (withstanding 100 MPa) in the Mariana Trench. This technology breaks the foreign technological blockade on full-ocean-depth power supplies, making my country the second country after Japan to master full-ocean-depth lithium power supply technology," Dong Shanmu said proudly.
Breakthrough from the basic design
The concept of lithium slurry battery technology was formally proposed by the Energy Storage Technology Research Group of the Institute of Electrical Engineering, Chinese Academy of Sciences, in a patent in 2015. A lithium slurry battery refers to a battery whose electrodes are entirely or partially composed of a slurry of lithium-storage active material, conductive agent, and electrolyte. Lithium slurry batteries have two significant technical characteristics: ultra-thick slurry electrodes and maintainable regeneration.
"Currently, the connection method used in lithium-ion batteries typically has a thickness of 100 to 200 micrometers. Increasing the thickness would cause severe cracking of the electrode sheets, electrode material detachment during use, reduced battery capacity, and decreased cycle performance," Liu Hao said.
Lithium slurry itself has a dynamic contact conductive network, eliminating the risk of detachment. Electrode thickness can reach millimeter levels, 10 to 50 times that of ordinary lithium-ion batteries. Therefore, lithium slurry batteries may be more suitable for providing high-capacity energy storage output. Lithium slurry batteries incorporate structural designs that, combined with the characteristics of the slurry electrodes, greatly facilitate electrolyte replenishment, electrolyte replacement, and slurry replacement operations.
"After a battery's performance declines over time, the internal interface can be repaired and regenerated to restore its vitality and extend its lifespan. In addition, when a battery is scrapped, the slurry is very easy to recycle, and the material can be used in the production of new batteries after regeneration," Liu Hao explained.
Liu Hao's research team has divided the evaluation of lithium slurry batteries and their processes into three stages. The first stage, the "60-point" stage, mainly verifies the performance and process feasibility of lithium slurry batteries, and this stage has been completed. The second stage, the "80-point" stage, aims to further improve the battery's energy density, enabling it to achieve a normal operating rate of over 0.2 and a transient rate of over 0.5, which is expected to be achieved by the end of this year. The "90-point" stage, to be reached in 2019-2020, will see product performance stabilize and gradually improve. The main work in this stage will be improving production technology, as well as subsequent online maintenance, recycling, and regeneration.
Do a good job in raw material recycling
"If we look at it from the perspective of the entire life cycle of lithium batteries, reducing the cost of battery materials requires starting with research and development on raw materials," said Li Li, a professor at Beijing Institute of Technology. "Raw material recycling can, to some extent, reduce the pressure on the national raw ore supply for the battery industry or other materials industries."
Currently, battery recycling technologies mainly include pretreatment processes, active methods, and wet methods. In recent years, many companies and research institutes have placed increasing emphasis on the pretreatment stage. Li Li explained that the dismantling and crushing processes primarily release volatile organic compounds from the electrolyte. Furthermore, the initial density screening directly impacts the leaching rates of various metals in the later stages.
Li Li stated that an important aspect of lithium battery recycling technology is analyzing the failure mechanism of lithium iron phosphate. "We hope to use different methods for each material, just like treating a patient with a specific ailment. Lithium iron phosphate has a very stable structure, and lithium positions will be missing after charging. Therefore, elemental analysis can reveal the function of lithium iron phosphate materials after failure or thousands of cycles."
“Materials such as iron phosphate and iron oxides directly reduce the performance and capacity of iron phosphate. Based on this failure mechanism, ‘lithium replenishment’ can be carried out in the later stage of the material mixture. This can be done in the form of carbonate or aluminum hydroxide, followed by high-temperature calcination, which can restore the material performance to some extent,” said Li Li.
In addition, traditional wet processing technology mainly uses a mixture of acids to separate the metal elements of materials from the solid phase to the liquid phase. This is mostly used for positive electrode materials, while the recovery of negative electrodes and electrolytes has been neglected in the past few decades.
“Perhaps because graphite for the negative electrode is very cheap, people feel it’s not worth replenishing,” Li Li expressed her opinion. “Currently, we hope to recycle and utilize different components in an all-round way. After recycling the negative electrode material, graphene material can be generated, and in the battery field or other fields, the high conductivity of graphene is very attractive.”
Electrolyte recovery is also technically feasible. By using a carbon dioxide-based extraction method, the electrolyte can be re-matched, and its conductivity can meet current commercial requirements.
However, how can we ensure high leaching efficiency, economic efficiency, and environmental friendliness of valuable metals in batteries? The initial material design is crucial. Could we design the materials from the very beginning to make them biodegradable?
"I hope to make some breakthroughs in the next few years," Li Li concluded.
