In our research on near‑space vehicles, the energy storage battery has emerged as a core component of the power system, directly determining the vehicle’s endurance, payload capacity, and overall mission effectiveness. The unique environment of near‑space—covering altitudes from approximately 20 km to 100 km—imposes stringent demands on these batteries, such as low pressure, extreme cold, and the need for high specific energy and long cycle life. Over the years, we have witnessed the transition from conventional lithium‑ion batteries to advanced lithium‑metal systems, and even to regenerative fuel cells, all aimed at meeting the ever‑increasing requirements of near‑space platforms. In this article, we present a comprehensive overview of the current application status and research progress of energy storage batteries for near‑space vehicles, with a strong emphasis on practical needs and key technological advancements.
1. Special Requirements for Energy Storage Battery in Near‑Space Environment
The near‑space environment is characterized by extremely low atmospheric pressure (down to 1.8 kPa at 20 km) and temperatures that can plunge below −60 °C. These conditions directly affect the behavior of the energy storage battery. For instance, the low external pressure can cause swelling or even rupture of soft‑pack cells if the internal vapor pressure is not properly managed. Moreover, the low temperature severely reduces the ionic conductivity of the electrolyte, leading to increased polarization and degraded capacity. To overcome these challenges, we must design energy storage batteries that can tolerate a wide temperature range (from ground ambient to −60 °C) and operate reliably under reduced pressure without gas generation. This requires careful selection of electrode materials and electrolytes that remain stable and highly conductive across the entire temperature window.
Another critical requirement is the specific energy of the energy storage battery. To enable a near‑space vehicle to stay aloft through the night, the battery must provide enough energy despite drastic weight limitations. Current platforms typically demand a specific energy exceeding 250 Wh/kg, and advanced systems already push this requirement to 400 Wh/kg or more. Additionally, the cycle life of the energy storage battery must reach hundreds of cycles (day‑night cycles) to support multi‑day or multi‑week missions. These demands drive the development of next‑generation chemistries and innovative battery architectures.
2. Application Status of Energy Storage Battery in Near‑Space Vehicles
2.1 Lithium‑Ion Batteries
Lithium‑ion batteries (LIBs) have been the most widely adopted energy storage battery for near‑space platforms, including the US Army’s “HISentinel” airship, Japan’s “SPF” stratospheric platform, and China’s “Yuanmeng” near‑space airship. The typical cathode materials used in these LIBs are lithiated transition metal oxides, whose key properties are summarized in Table 1.
| Cathode Material | Voltage Range (V) | Specific Capacity (mAh/g) | Cycle Performance |
|---|---|---|---|
| LiCoO₂ | 3.0 – 4.4 | 140 – 170 | Moderate |
| LiMnO₂ | 3.0 – 4.2 | 110 – 140 | Good |
| LiFePO₄ | 2.0 – 3.6 | 140 – 160 | Excellent |
| Li(MnCoNi)O₂ | 2.7 – 4.4 | 120 – 220 | Excellent |
| xLi₂MnO₃·(1−x)LiMO₂ | 2.0 – 4.8 | 250 – 300 | Moderate |
LIBs for near‑space applications benefit from a high operating voltage (averaging above 3.7 V) and good cycle stability, especially for LiFePO₄ and ternary materials. However, the specific energy of commercial LIBs typically ranges from 200 to 300 Wh/kg, which is becoming insufficient for next‑generation missions. This limitation has prompted the adoption of lithium‑metal batteries.
2.2 Lithium‑Metal Batteries
Lithium‑metal batteries, which use metallic lithium as the anode, represent a major leap forward because lithium offers an extremely high theoretical specific capacity (3860 mAh/g) and the lowest electrochemical potential (−3.04 V vs. SHE). As a result, the energy density of a single cell can reach 450–500 Wh/kg. We have developed a high‑specific‑energy lithium‑metal energy storage battery exceeding 500 Wh/kg, with stable cycling over 200 cycles. In June 2022, this battery successfully completed a 7‑day near‑space flight at an altitude of 20 km and a night‑time altitude of 10 km. Other players such as Solid Energy Systems (SES) and Northvolt have also reported lithium‑metal cells with ~400 Wh/kg and long cycle life, targeting near‑space drones and high‑altitude platforms.
2.3 Other Energy Storage Batteries
Beyond lithium‑based batteries, regenerative fuel cells (RFCs) have attracted attention as a promising candidate for ultra‑long endurance missions. RFCs combine hydrogen‑oxygen fuel cells with water electrolysis, achieving a theoretical specific energy of 400–1000 Wh/kg with no self‑discharge and unlimited depth of discharge. When integrated with solar panels, RFCs can store surplus energy during the day and release it at night, forming a complete day‑night energy system. Although still in the experimental stage, such systems have already been demonstrated on small UAVs like the US “Ion Tiger” and China’s “Thunderbird” hydrogen‑fuel‑cell UAV.
3. Research Progress in Key Technologies for Energy Storage Battery
To further improve the performance of energy storage batteries for near‑space vehicles, research efforts have focused on modifying the cathode, anode, and electrolyte. Below we highlight the most recent advances.
3.1 High‑Ni Ternary Cathode Materials
The specific capacity of Ni‑rich cathodes (e.g., LiNiₓCoᵧMn₁₋ₓ₋ᵧO₂) can exceed 200 mAh/g, but they suffer from structural instability, Li/Ni mixing, and surface side reactions. To address these issues, we have adopted strategies such as composition gradient design, elemental doping, and surface coating. For example, a coherent perovskite phase introduced into the cathode can “pin” the lattice and suppress volume changes during cycling, leading to a capacity retention of 94.7% after 200 cycles. Doping with Mg²⁺ (as in Li₀.₉₈Mg₀.₀₂Ni₀.₉₄Co₀.₀₆O₂) reduces anisotropic lattice distortion and improves thermal stability, delivering 214 mAh/g with 80.1% retention after 500 cycles in a pouch cell.
Surface coating with fast‑ion conductors, such as Li₀.₁₂₅La₀.₆₂₅TiO₃ (LLTO), can lower the Li‑ion migration activation barrier and increase the diffusion coefficient by an order of magnitude, as shown by the following comparison:
$$
\text{Coated: } D_{\text{Li}} \approx 2\times10^{-12} \ \mathrm{m^2/s},\quad \text{Uncoated: } D_{\text{Li}} \approx 2\times10^{-13} \ \mathrm{m^2/s}.
$$
Combined approaches—doping, gradient concentration, and coating—have been successfully employed to produce Ti‑doped full‑concentration‑gradient LiNi₀.₇₅Co₀.₁₅Mn₀.₁₀O₂ with a Li₂ZrO₃ coating, maintaining 88.1% capacity after 200 cycles at 55 °C. These improvements are crucial for the energy storage battery used in prolonged near‑space missions.
3.2 Lithium‑Metal Anode
The high reactivity of lithium metal leads to dendrite formation and continuous side reactions with the electrolyte. To stabilize the lithium anode, we have explored electrolyte engineering, artificial SEI layers, and composite hosts. For instance, using tetrahydrofuran‑based solvents such as ethyl‑1,1,2,2‑tetrafluoroethyl ether promotes the formation of a LiF‑rich SEI with high mechanical strength. High‑concentration electrolytes (e.g., 4 M LiFSI in DME) yield a nodular, dendrite‑free lithium morphology, achieving an average Coulombic efficiency of 98.4% over 1000 cycles. The schematic effect of lithium polysulfide and LiNO₃ additives is illustrated by the formation of a stable SEI containing Li₃N and Li₂S, which effectively suppresses dendrite growth.
Artificially introducing a protective LiF layer on the lithium surface (via F₂ gas treatment) results in a 380 nm thick film that enables stable cycling at 1 mA/cm² for 300 cycles without dendrites. Additionally, we have developed a Li‑3D‑host composite by infiltrating molten lithium into a nickel foam scaffold. The exposed nickel tips act as nucleation sites, guiding uniform lithium deposition and avoiding volume expansion. The resulting lithium‑metal energy storage battery shows excellent cycle stability and a flat surface morphology.
3.3 Electrolyte Research
The electrolyte is a key enabler for both low‑temperature operation and long cycle life of the energy storage battery. We have investigated lithium salts such as LiDFOB and LiFSI. LiDFOB decomposes to form flexible oligomers in the SEI, providing high Coulombic efficiency (95–97%) in Li‑Cu cells. High‑concentration LiFSI (4 M in DME) gives a unique nodular lithium deposition morphology, as shown by SEM, which minimizes damage to the separator. The organic solvent also plays a critical role. Linear carbonates (e.g., VC) can form a double‑layer SEI with an inner inorganic layer and an outer poly‑VC layer, exhibiting an average Young’s modulus of 34 GPa. This mechanically robust SEI allows Li/Li symmetric cells to cycle for over 4000 h at high current densities.
For high‑voltage applications, we designed a localised high‑concentration ether‑based electrolyte (LiFSI/DME‑TTE) that works well with NCM811 cathodes up to 4.5 V. The fluorinated diluent TTE reduces viscosity and participates in the formation of stable passivation layers on both the lithium anode and the cathode. Under C/3 cycling, the Li/NCM811 cell retained 82% capacity after 250 cycles, demonstrating that proper electrolyte design can simultaneously satisfy the demands of high energy density and long cycle life for near‑space energy storage battery.
4. Conclusion
In summary, the energy storage battery is a vital element of near‑space vehicles, directly influencing their endurance, payload, and mission success. While lithium‑ion batteries currently dominate the field, the increasing demand for higher specific energy and longer cycle life is driving the development of lithium‑metal batteries and regenerative fuel cells. Through continuous research on high‑Ni cathodes, lithium‑metal anodes, and advanced electrolytes, we have achieved significant improvements in cycle stability, low‑temperature performance, and energy density. The joint advancement of vehicle design and energy storage battery technology will accelerate the exploration and exploitation of near‑space, enabling long‑endurance platforms for both military and civilian applications.