A Solid-Sate Battery Technology is a type of battery that utilizes a solid-state electrolyte instead of a liquid electrolyte. Lithium batteries consist of various components such as positive electrodes, negative electrodes, diaphragms, electrolytes, collecting fluids (conductive metal foils), and packaging materials. The core components are the positive and negative electrodes along with the diaphragm electrolytes. In power batteries, the main materials for the positive electrodes are lithium iron phosphate and lithium ternary, while the negative electrode is graphite or a combination of graphite and silicon. The materials and quantities used for the positive and negative electrodes determine the battery’s capacity.
The energy of the battery is derived from the back-and-forth movement of lithium ions between the positive and negative terminals. The positive and negative electrodes can be likened to sponges that can absorb or release lithium ions. During charging, driven by the electric potential of the external power supply, lithium ions move from the positive electrode to the negative electrode and become embedded in the carbon gaps of the negative electrode. The lithium ions possess high potential energy, and this process converts electrical energy into chemical energy. During discharging, the lithium ions spontaneously move from a high-energy state (negative electrode) to a low-energy state (positive electrode), simultaneously releasing energy and converting chemical energy into electrical energy. Therefore, the number and capacity of the positive and negative electrodes (sponges) determine the upper limit of the battery’s energy.
From an application perspective, having more energy is desirable; however, more energy also means increased risks and instability. Particularly, when there is still a liquid electrolyte present between the positive and negative terminals, it poses the greatest risk to the battery. Current technology involves adding certain substances to the positive electrode to enhance its ability to absorb lithium, and adding silicon to the negative electrode can also improve its lithium absorption capacity. However, it is not the strengths that determine the upper limit of performance, but rather the weaknesses.
The electrolyte situated between the positive and negative electrodes is typically in a liquid state, hence its name. Its primary function is to facilitate ion conduction. The diaphragm serves to isolate the positive and negative electrodes, preventing direct contact while allowing the passage of only lithium ions. The electrolyte is primarily composed of organic compounds, particularly ether and aldehyde groups, which exhibit active properties, flammability, and chemical reactivity. The diaphragm is a thin layer of organic material that is relatively fragile and susceptible to puncture. If the diaphragm is punctured, resulting in a short circuit between the positive and negative electrodes, it can easily lead to a fire.
A solid-state battery addresses this issue by converting the electrolyte from a liquid state to a solid state, aiming to compensate for this weakness. The all-solid-state battery represents a significant advancement in solid-state battery technology, effectively addressing this weakness to a great extent.
Advantages of solid-state batteries: Safety and high energy density
One of the major drawbacks of liquid electrolytes, as mentioned earlier, is that despite not being the most active component in the battery, their dual identity as both a liquid and a hazardous organic material makes them a weak point in the battery. Being an organic substance, it is prone to catching fire and burning, and being a liquid, it is susceptible to leakage, spontaneous combustion, and the effects of external forces such as collisions. Additionally, the diaphragm, a thin layer of organic material, faces challenges in finding the right balance in thickness. If it is too thick, it hinders the movement of lithium ions back and forth, and if it is too thin, it lacks the strength to resist punctures. During use, it cannot withstand punctures caused by lithium dendrites (lithium ions growing and puncturing the electrolyte).
Technical difficulties: Solid interface and transmission resistance problems
If we simply change the electrolyte from liquid to solid without making any modifications to the existing lithium iron phosphate and ternary lithium batteries, it would not lead to improved performance. Instead, performance would be compromised in various aspects. Two main reasons account for this:
Solid interface problem
In a battery, the positive and negative electrodes are solid materials. When a liquid electrolyte is present between them, the liquid can make good contact and wet the solid surfaces. Although the solid-liquid interface energy is higher than that inside the liquid, lithium ions can still transfer smoothly from the positive (solid) to the liquid and from the liquid to the negative (solid). This process can be imagined as sugar dissolving in water. However, when the middle layer is solid, the situation changes. The contact between solid interfaces cannot be as effective as the solid-liquid interface. Visualize two boards being pressed together. No matter how smooth and tightly fitted they are, there will always be a gap in the middle, separating the molecular and atomic levels of the two surfaces. It becomes challenging for lithium ions to pass through this gap, leading to an accumulation of ions at the interface.
Solid transmission resistance problem
Lithium ions need to shuttle back and forth between the positive and negative terminals. When a liquid is present in the middle, the resistance is minimal. However, this resistance increases when transitioning to a solid state. Consider the difference between swimming in water and swimming in sand. This is determined by the physical and chemical properties of solids. While we can find ways to optimize this resistance, it cannot be completely eliminated. Lithium ions face significant obstacles in solid-state batteries. Starting from the positive terminal, they struggle to pass through the solid interface of the positive electrode-electrolyte, then encounter difficulties traversing the solid electrolyte. Finally, they face challenges in crossing the solid interface of the negative electrode-electrolyte and reaching the negative terminal. The journey is bumpy and arduous, and they must surely envy their liquid counterparts.
The technological breakthrough direction of solid-state batteries
Building upon the basic principles, researchers have proposed methods and measures for improvement. Even if the problems cannot be fully solved, optimization and mitigation can still be achieved, which is sufficient.
Thinning the electrolyte: Since solids are not easily traversed, reducing thickness always reduces the difficulty.
Increasing lithium concentration in the electrolyte: When the lithium concentration is very high, in extreme cases, lithium entering from the left side can displace a neighboring lithium atom on the right side. It eliminates the need to traverse the entire electrolyte. Squeezing from the opposite side secures a win. (This analogy, though optimistic, may not reflect the actual situation accurately; it is merely illustrative.)
In situ growth: Solid electrolytes grow on the surfaces of the positive/negative electrodes. They intertwine and become indistinguishable. The interface still exists, but it is no longer as pronounced as before. This mutual combination of different materials at the atomic level is fundamentally different from the melting combination of different metals. Technology is highly challenging.
Retaining a small amount of liquid electrolyte: A thin layer of liquid is preserved between the positive electrode and the solid electrolyte. Lithium ions initially jump into the liquid before entering the solid phase. Although an extra step is involved, it significantly reduces the interface, making it a practical strategy (referred to as a quasi-solid-state battery). Personally, I believe that it is at this stage that the true potential of Solid-State Battery Technology is realized.
The application prospects are promising, but there are current challenges.
Solid-state batteries have gained widespread recognition in the industry. Many global car companies and battery manufacturers have announced plans for solid-state batteries. Companies such as BMW Manufacturing Co., LLC, Volkswagen, Renault Nissan Motor Corporation, Toyota Motor Corporation, Honda Cars India Ltd, General Motors, and Ford Motor Company, as well as domestic new players, have corresponding plans or investment strategies. NIO, for example, intends to sell semi-solid-state batteries this summer, although they come at a high price. Domestic new players and car companies like SAIC Motor, Dongfeng Automobile Co., Ltd., BAIC Motor Corporation Limited( 北京汽车股份有限公司), and Ideal have relevant strategies in place, relying heavily on suppliers’ research and development progress, such as Contemporary Amperex Technology Co., Limited, BYD, QINGTAO FU, WEINENG MAOYIYOU Trading co., GanfengLithium, and others. Once the battery technology matures, it will be time for implementation and mass production. However, all-solid-state battery technology is currently too immature for practical application.
Semi-solid-state battery technology is relatively simpler and is expected to appear gradually over the next few years. The large-scale application of solid-state batteries will happen sooner or later. Currently, the main limitations for implementation are performance and cost. Performance deficiencies stem from immature technology, and fundamental breakthroughs are yet to be achieved in addressing the aforementioned technical difficulties. The cost is influenced by the small number of solid-state electrolytes and the associated technical challenges.
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