In recent years, the demand for high-energy density lithium batteries has continued to rise in fields such as electric vehicles, aerospace, and large-scale energy storage. Although traditional commercial lithium-ion batteries are widely used in consumer electronics and light transportation, they are difficult to simultaneously meet the industry's demands for higher energy density, longer lifespan, and more stringent environmental stability. To further improve the energy density of batteries, it is necessary to work together from the aspects of core material system (positive electrode, negative electrode, electrolyte) and overall packaging design.
At present, there are two main technological routes in the industry to improve energy density: one is to make extreme improvements to liquid lithium batteries, including higher nickel content positive electrodes, silicon-based or lithium metal negative electrodes, thinner or even no separators, etc; The second is all solid state or "quasi solid state" technology, which replaces traditional liquid electrolytes to achieve better volume utilization and higher safety thresholds. However, the former faces challenges such as poor interface stability and rapid capacity decay, while the latter has not yet fully broken through in terms of large-scale production processes, material compatibility, and cost control. In addition, differentiated requirements have been put forward for battery design based on the load and range requirements of different application scenarios (such as new energy vehicles, drones, aircraft, etc.): some places emphasize power density and safety, while others focus more on extreme specific energy to extend range and reduce total weight.
1. Theoretical basis and design ideas
1.1 Theoretical upper limit and limiting factors of energy density
When designing high-energy density lithium batteries, it is necessary to first identify the key factors that affect the energy density (Wh/kg or Wh/L) of the battery cell, including the specific capacity of the positive and negative electrode materials, operating voltage, electrode ratio (N/P ratio), proportion of active materials, and packaging structure.
At the material level, high-capacity positive electrodes (such as lithium manganese rich, NCM811, and even Li-O2 systems with ultra-high theoretical capacity) and high-capacity negative electrodes (silicon carbon, pure lithium metal, or metal alloys) can significantly improve the energy density of individual cells, but both may encounter bottlenecks in terms of cycle life and safety;
Interface and side reactions: High energy density systems often mean more demanding operating voltages and more compact structures, making the electrode/electrolyte interface prone to unstable side reactions such as gas generation and metal ion dissolution;
Component design: Ultra thin or even eliminating membranes, thinning current collectors (copper foil, aluminum foil), or using lightweight packaging can reduce the proportion of inactive mass, but at the same time, higher requirements are placed on manufacturing processes and safety control.
In many research and commercialization cases, battery design can be summarized as a layered strategy: first set a target energy density (such as 500 Wh/kg, 700 Wh/kg, or even 1000 Wh/kg), and then deduce material system and structural parameters, such as positive and negative electrode load, proportion of active material, electrode thickness, separator type, etc. As the target value increases, the material system often evolves from graphite/NCM811 to Si-C/high nickel NCM, then to Li metal/lithium rich positive electrode, and finally extends to cutting-edge forms such as all solid state batteries or lithium sulfur, lithium air, etc.
1.2 Liquid to Solid State: Evolution and Challenges
The paper provides an overall overview of the technological evolution from liquid to all solid state:
High energy liquid batteries: Ultra high nickel NCMs (such as NCM9 series) are commonly used, combined with artificial or functional coating separators and ultra-thin negative electrode coatings to reduce irreversible losses. Some schemes even introduce local solid electrolytes to improve the safety factor;
Quasi solid state battery: use gel or some solid electrolytes mixed with liquid electrolytes to maintain relatively high ionic conductivity, and also to improve the dendrite problem caused by excessive lithium deposition on the negative side;
All solid state batteries: completely replacing liquid electrolytes with solid electrolytes (sulfides, oxides, or polymers) can significantly increase energy density and resist higher voltage and high temperature environments, but large-scale manufacturing and interface contact are still technical difficulties.
In principle, the all solid state solution is more sensitive to material purity and preparation process, and requires complete densification under high pressure/hot pressing environment to achieve sufficient ion conductivity and close interface contact. Meanwhile, lithium negative electrodes are prone to interface reactions such as high impedance interface layer (SCL) or stress-induced cracks under all solid state conditions, which will limit their cycle life and rate performance.
2. Material System: Positive Electrode, Negative Electrode, and Electrolyte
2.1 High nickel positive electrode and lithium rich positive electrode
(1) High nickel ternary (NCM, NCA)
The high nickel system (NCM811, NCM9 series) has become the mainstay of liquid high-energy batteries at present due to its reversible capacity of 200+mAh/g. However, when the nickel content is further increased, the structural stability, thermal stability, and interface side reactions will deteriorate. The literature proposes a series of solutions, including surface coating (such as Al ₂ O3, ZrO ₂), doping (such as Mg, Al), and single crystal structure, to suppress phase transition and microcrack formation, thereby extending cycle life.
(2) Rich lithium manganese based/rich lithium oxide
Rich lithium manganese based materials (Li ₁ ₂Mn₀. ₅₅Ni₀. ₁₅Co₀. The theoretical capacity of (₁₀₂, etc.) can exceed 300 mAh/g, and even reach over 350 mAh/g, but there are problems such as severe irreversible capacity in the first week, voltage fade, and low rate performance, which require more refined research and development in particle morphology, doping, and surface modification. The article discusses how combining such "lithium rich cathodes" with lithium metal or silicon-based cathodes and stacking them with all solid state electrolytes may lead to finding new equilibrium points in the energy density range of 700-800 Wh/kg or even higher.
2.2 Negative electrode: from graphite to silicon-based and then to lithium metal
(1) Graphite and its modification
Traditional graphite negative electrodes have advantages such as stable cycling and mature technology, but their specific capacity (about 372 mAh/g) is no longer sufficient to meet higher energy density requirements. Proper addition of silicon micro powder or silicon oxide can increase capacity, but it also brings about expansion and side reactions.
(2) Silicon based negative electrode
The theoretical specific capacity of silicon-based negative electrode can reach over 3500 mAh/g. If it can effectively suppress volume expansion and maintain stable SEI film, the energy density can be significantly improved. Some commercial batteries have attempted to incorporate 5-10% silicon into the negative electrode to increase capacity. However, special attention still needs to be paid to the interface matching with solid-state electrolytes, expansion stress, and maintenance of conductive networks in silicon-based environments.
(3) Lithium metal
In an ideal state, the theoretical capacity (3860 mAh/g) and working potential of the lithium metal negative electrode are close to 0 V, which will significantly improve the energy density of the entire package. However, due to the growth of dendrites, volume changes, and interface side reactions, lithium metal batteries in liquid systems are mostly in the laboratory stage. Solid state electrolytes can to some extent suppress dendrite expansion and reduce side reactions, but they require extremely high process requirements and still need to solve the problems of "elastic matching" and "full life safety".
2.3 Electrolyte: from liquid, organic gel to solid
Liquid electrolyte: High voltage stability is often required for high-energy batteries, and the addition of phosphate or other new additives can enhance interface stability. However, as the voltage increases to 4.5-4.8 V, side reactions and gas release become more prominent;
Polymer electrolyte: It has plasticity and certain safety, but its ionic conductivity is difficult to match that of liquid state, and is mostly used in medium or high temperature scenarios;
Sulfide solid electrolyte: Representative materials such as Li ₁₀ GeP ₂ S ₁₂ (LGPS) have ion conductivity comparable to that of liquid state, but are extremely sensitive to humid environments and prone to issues such as H ₂ S generation;
Oxide solid electrolytes, such as LLZO (Li ₇ La ∝ Zr ₂ O ₁ ₂), have excellent stability and low sensitivity to air, but the densification sintering temperature is high and the interface impedance is difficult to control.
The literature points out that different solid electrolytes are suitable for different scenarios, and it is difficult for a "perfect material" to fully dominate the market in the short term. The key still depends on the specific application (automotive, aviation, or energy storage) and production line process conditions.
3. Structural Design and Component Optimization of High Energy Density Batteries
3.1 Stacking/winding and pole thickness
Whether it is a liquid or solid state battery, the cell structure is often assembled by stacking or winding. To achieve high energy density, it is necessary to increase the polar load and reduce the ineffective volume. However, excessive load can easily lead to poor internal ion transport, increased polarization, and increased heat generation. Therefore, the paper suggests optimizing parameters such as N/P ratio and electrode compaction density to balance the positive and negative electrode capacities while avoiding uneven conduction caused by excessively thick electrode plates.
3.2 Diaphragm, current collector and packaging
Diaphragm: Ultra thin or functionally coated separators are often used in high-energy batteries, and even solid-state batteries may eliminate traditional separators. But to ensure safety and stable ion pathways, a balance needs to be found between "thickness" and "puncture resistance";
Current collector: Reducing the thickness of aluminum foil and copper foil or replacing them with lighter, high-strength metal foil is an important means of reducing inactive weight;
Packaging and thermal management: As capacity and energy increase, thermal management becomes more critical. Although all solid state batteries have a higher temperature threshold for thermal runaway, they still need to improve their heat dissipation and mechanical buffering structures.
4. Manufacturing Process and Feasibility Study
4.1 Extreme improvement of liquid batteries
To achieve a liquid system of 500 Wh/kg or more on a conventional production line, efforts are usually made in the following areas:
High load electrodes (>4-5 mAh/cm ²) require strict requirements for coating uniformity and drying processes;
Ultra thin membranes and lightweight current collectors, such as 5 µ m copper foil, 9 µ m aluminum foil, 12 µ m or even 9 µ m membranes;
N/P ratio: Reduce excess negative electrode appropriately;
Low electrolyte addition: Reduce residual liquid through tape or vacuum infiltration process.
Through this "digging to the limit" approach, some companies can produce 18650/2170 cylindrical or pouch batteries with an energy density of approximately 350-400 Wh/kg in specific environments, but their cycle life and safety protection need to be further optimized.
4.2 Difficulties in Solid State Process
Solid state electrolyte preparation: Sulfides require an inert and dry environment, while oxides require high-temperature sintering and are difficult to prepare;
Stacked pressing: It is often carried out under high pressure (>100 MPa), and sufficient contact between particles must be ensured;
Negative electrode treatment: If using lithium foil or ultra-thin lithium, on the one hand, it is necessary to avoid contact with water and oxygen, and on the other hand, the foil material itself is prone to breakage or wrinkling.
Although all solid state technology can theoretically achieve astonishing energy densities of 600-1000 Wh/kg, the difficulty and cost of mass production remain high. The literature points out that in order to achieve large-scale application of all solid state batteries in the next 5-10 years, it is necessary to continuously deepen research in material synthesis, mechanized molding, interface engineering, and cycle management.
5. Application prospects: from electric vehicles to aircraft
The paper emphasizes that the potential applications of high-energy density batteries are not limited to electric vehicles, but also include unmanned aerial vehicles (UAVs), electric vertical takeoff and landing vehicles (eVTOLs), small manned aircraft, and spacecraft. These scenarios require higher energy density and specific power of the battery, as well as more stringent restrictions on safety and volume.
Drones and short haul aircraft: High nickel based liquid batteries with silicon-based negative electrodes or transitioning to quasi solid state batteries may be preferred to achieve longer endurance while ensuring safety;
Large passenger aircraft: Currently, it is still difficult to rely entirely on battery power, but "battery+fuel cell" hybrid or "hybrid" solutions are gradually emerging. Once all solid state or ultra-high energy battery technology matures, aviation emissions reduction and safety will benefit greatly.
In addition, the article briefly mentions that in the field of large-scale energy storage (wind power, photovoltaic grid connection), high energy density can reduce land occupation and construction costs. If safety and cost can be achieved simultaneously, the all solid state route also has considerable potential.
6. Overview of Key Innovations and Challenges
Through the summary and analysis of the paper, it can be seen that the author proposes a series of systematic thinking and route selection for the design of liquid and all solid state high-energy batteries:
Material and structure coupling: from positive and negative electrode active materials to electrolytes and packaging, each component is closely related;
Phased evolution: first limit upgrade liquid technology, then gradually transition to gelled or quasi solid state, and finally move to all solid state;
The "safety performance cost" triangle balance: finding the optimal midpoint between ultra-high specific energy and economic feasibility;
Scenario customization: Establish the optimal material combination for different energy levels (200 Wh/kg~1000 Wh/kg) and application scenarios (passenger cars, aircraft, energy storage).
The core challenges come from the materials themselves, such as lithium rich positive electrode voltage decay, silicon negative electrode expansion, and solid-state interface problems; This is also due to the scale of the process and cost limitations, such as the preparation of ultra-thin electrode sheets and consistency control.