The relentless pursuit of human exploration, particularly the ambitious goal of establishing a sustained presence on the Moon, presents formidable challenges for supporting technologies. One of the most critical is the development of robust energy storage systems capable of surviving and operating in the profound cold of extraterrestrial environments. On the lunar surface, devoid of a protective atmosphere, temperatures during the extended lunar night can plummet to a staggering -180°C. This extreme thermal environment necessitates power sources that are not only highly energy-dense and reliable but also functionally resilient at temperatures far below the operational limits of conventional systems. The li ion battery, with its established pedigree of high specific energy, long cycle life, and proven reliability, has become the cornerstone of power systems for countless terrestrial and space applications. However, its widespread adoption for future lunar and deep-space missions is currently hindered by a significant performance degradation at temperatures below approximately -20°C. This decline manifests as severely reduced usable capacity, power capability, and often a complete inability to accept charge, primarily due to exacerbated electrochemical polarization and a dramatic increase in internal resistance. Therefore, pioneering research into enabling li ion battery operation at ultra-low temperatures (below -40°C) is not merely an academic exercise but a critical engineering imperative for the future of space science and exploration. This article delves into the fundamental challenges and presents a comprehensive experimental investigation into optimizing both electrode materials and electrolyte formulations to unlock the potential of the li ion battery in the coldest frontiers.
The performance of a li ion battery at any temperature is a complex interplay between kinetics and thermodynamics. As temperature decreases, several interrelated factors conspire to degrade performance. The voltage ($V_{cell}$) during operation can be expressed as a function of its open-circuit voltage ($E_{OCV}$), overpotential due to polarization ($\eta$), and the voltage drop across the internal resistance ($R_{int}$):
$$ V_{cell} = E_{OCV} – |\eta| – I \cdot R_{int} $$
At ultra-low temperatures, both $|\eta|$ and $R_{int}$ increase drastically, leading to rapid voltage cut-off and minimal accessible capacity. The polarization ($\eta$) itself is a sum of contributions: ohmic polarization ($\eta_{ohm}$) from sluggish ion transport in the electrolyte, concentration polarization ($\eta_{conc}$) from slow diffusion of ions in the electrolyte and active materials, and activation polarization ($\eta_{act}$) associated with the charge transfer reaction at the electrodes.
$$ \eta = \eta_{ohm} + \eta_{conc} + \eta_{act} $$
The ionic conductivity ($\sigma$) of the electrolyte, a key determinant of $\eta_{ohm}$, follows an Arrhenius-type relationship:
$$ \sigma = A \cdot e^{-E_a/(RT)} $$
where $E_a$ is the activation energy for ionic conduction, $R$ is the gas constant, and $T$ is the absolute temperature. As $T$ plummets, $\sigma$ drops exponentially. Simultaneously, the increase in electrolyte viscosity severely hampers ion diffusion, elevating $\eta_{conc}$. Perhaps most critically at very low temperatures, the charge transfer resistance ($R_{ct}$), linked to $\eta_{act}$, becomes the dominant impedance. This $R_{ct}$ is heavily influenced by the energy barrier for the desolvation of Li+ ions from their solvation sheath in the electrolyte before they can cross the solid-electrolyte interphase (SEI) and intercalate into the anode. This desolvation process becomes exponentially slower as thermal energy is removed. Furthermore, the contraction of electrode materials and conductive networks can increase electronic contact resistance. Our research strategy, therefore, focuses on a dual-path approach: engineering anode materials with superior low-temperature kinetics and designing electrolytes with high ionic conductivity and favorable interfacial properties to mitigate these fundamental barriers.
Experimental Design and Methodology
To systematically investigate pathways for ultra-low temperature performance, we fabricated 10 Ah pouch cells based on a Lithium Cobalt Oxide (LiCoO2) cathode and a hard carbon (HC) anode. A baseline configuration (Scheme 1) was established. Subsequently, targeted modifications were applied to the anode composition and the electrolyte formulation to evaluate their individual and combined effects. The detailed experimental schemes are summarized in the table below.
| Scheme ID | Anode Composition | Electrolyte Formulation |
|---|---|---|
| 1 (Baseline) | 100% Hard Carbon (HC) | 1.0 M LiPF6 in PC/EMC (1:2 by wt.) |
| 2 | 80% HC + 20% Graphite | 1.0 M LiPF6 in PC/EMC (1:2 by wt.) |
| 3 | 50% HC + 50% Graphite | 1.0 M LiPF6 in PC/EMC (1:2 by wt.) |
| 4 | 100% HC | 1.0 M (0.8LiPF6 + 0.2LiPO2F2) in PC/EMC |
| 5 | 100% HC | 1.0 M (0.8LiPF6 + 0.2LiFSI) in PC/EMC |
| 6 | 100% HC | 1.0 M LiPF6 in PC/EMC + 1% DTD |
| 7 | 100% HC | 1.0 M LiPF6 in FEC/PC/EMC |
| 8 (Optimized) | 100% HC | 1.0 M (0.8LiPF6 + 0.2LiFSI) in FEC/PC/EMC + 1% DTD |
All cells underwent standardized formation cycles at room temperature. Their low-temperature performance was then rigorously evaluated, with a focus on charge acceptance at -40°C and discharge capability at -50°C. Electrochemical impedance spectroscopy (EIS) and conductivity measurements of bulk electrolytes were conducted across a wide temperature range to correlate formulation changes with fundamental property improvements.
Anode Engineering: The Key to Low-Temperature Charging
The ability of a li ion battery to be safely and effectively charged at ultra-low temperatures is paramount for many applications, including lunar rovers that must replenish energy during cryogenic periods or systems requiring resilience against thermal faults. The charging process at low temperature is primarily limited by the anode’s kinetics. Severe polarization can drive the anode potential below 0 V vs. Li/Li+, leading to metallic lithium plating. This side reaction irreversibly consumes cyclable lithium and electrolyte, increases impedance, and poses a severe safety risk due to dendrite growth. Traditional graphite anodes, with their highly ordered layered structure, suffer from slow Li+ diffusion and significant lattice contraction at low temperatures, making them particularly susceptible.
We investigated the use of hard carbon as a low-temperature-friendly anode material. Unlike graphite, hard carbon possesses a turbostratic structure with larger interlayer spacing ($d_{002} > 0.34$ nm), abundant nanopores, and short-range order. This structure allows for faster Li+ diffusion from multiple directions and reduces the strain associated with (de)intercalation. To quantify this benefit, we compared the -40°C, 0.2C charging performance of pure HC anode (Scheme 1) with composites containing graphite (Schemes 2 & 3).
| Anode Composition | Charge Capacity Retention at -40°C, 0.2C (%) | Key Observation |
|---|---|---|
| 100% Hard Carbon | 85.99 | Highest capacity, most stable voltage profile. |
| 80% HC + 20% Graphite | 82.90 | Moderate performance drop. |
| 50% HC + 50% Graphite | 64.32 | Severe performance degradation, high polarization. |
The results are unequivocal: the pure hard carbon anode delivers superior low-temperature charge acceptance. The charging curves for the graphite-containing cells show significantly higher polarization and earlier voltage cut-off. The capacity retention plunges from ~86% for pure HC to ~64% for the 50/50 composite. This demonstrates that while graphite offers high capacity and good performance at room temperature, its kinetics become prohibitively slow in the extreme cold. Hard carbon, with its more open and disordered structure, maintains viable Li+ transport pathways, making it an essential component for li ion battery designs targeting ultra-low temperature operability, especially for charging.
Electrolyte Formulation: The Battle Against Rising Impedance
While the anode governs chargeability, the electrolyte is the central nervous system of the li ion battery, critically influencing both charge and discharge performance at all temperatures. For ultra-low temperature operation, the electrolyte must simultaneously achieve high ionic conductivity, low viscosity, a low melting point, and the ability to form a stable, low-impedance SEI. We systematically deconstructed the electrolyte, examining the roles of the lithium salt, functional additives, and solvent system.
Lithium Salt Selection and Conductivity
The choice of lithium salt directly impacts the Li+ solvation structure, dissociation constant, and the nature of the SEI. We compared the baseline LiPF6 salt with blends incorporating lithium difluoro(oxalato)borate (LiDFOB or LiODFB, approximated as LiPO2F2 in the original text) and lithium bis(fluorosulfonyl)imide (LiFSI). The discharge performance at -50°C and 0.5C is shown below, with key metrics summarized in Table 3.
| Lithium Salt | Capacity Retention at -50°C, 0.5C (%) | Discharge Curve Inflection Voltage (V) |
|---|---|---|
| 1.0 M LiPF6 (Baseline) | 68.83 | 2.62 |
| 1.0 M (0.8LiPF6+0.2LiPO2F2) | 72.62 | 2.66 |
| 1.0 M (0.8LiPF6+0.2LiFSI) | 75.99 | 2.83 |
The addition of both LiPO2F2 and LiFSI improved low-temperature capacity, with LiFSI providing the most significant boost. This can be attributed to the unique properties of the FSI– anion. It has a highly delocalized negative charge, leading to a weaker Coulombic interaction with Li+ compared to PF6–. This weaker association energy ($E_a$ in the conductivity equation) facilitates easier Li+ dissociation and transport, resulting in higher ionic conductivity, especially at low temperatures. Bulk electrolyte conductivity measurements corroborate this (Table 4). The blend with 20% LiFSI consistently shows the highest conductivity across the entire temperature range, being over twice as conductive as the baseline at -50°C.
| Scheme (Salt) | Ionic Conductivity (mS/cm) at Temperature (°C) | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| -50 | -40 | -30 | -20 | -10 | 0 | 10 | 20 | 30 | ||
| 1 (LiPF6) | 0.34 | 0.74 | 1.33 | 2.17 | 2.94 | 3.92 | 5.78 | 6.66 | 7.69 | |
| 4 (LiPF6/LiPO2F2) | 0.59 | 1.41 | 2.70 | 3.23 | 3.57 | 5.11 | 5.66 | 6.89 | 8.13 | |
| 5 (LiPF6/LiFSI) | 0.74 | 1.61 | 2.20 | 3.58 | 4.45 | 5.11 | 6.18 | 7.10 | 8.69 | |
The higher inflection voltage for the LiFSI-containing cell (2.83 V vs. 2.62 V for baseline) also indicates lower overall polarization during discharge, meaning more usable energy can be delivered before hitting the lower voltage cut-off limit. This makes LiFSI a highly effective co-salt for ultra-low temperature li ion battery electrolytes, despite potential challenges with aluminum current collector corrosion at high voltages which must be managed in full-cell design.
The Role of Film-Forming Additives: 1,3,2-Dioxathiolane 2,2-dioxide (DTD)
Additives are potent tools for engineering the electrode-electrolyte interface. DTD is known to participate in the formation of the SEI on the anode, modifying its composition and structure to be more conductive and stable. As shown in Table 5, adding just 1% DTD to the baseline electrolyte (Scheme 6) improved the -50°C discharge capacity retention from 68.83% to 72.68%. More strikingly, the inflection voltage rose significantly to 3.02 V, indicating a substantial reduction in polarization. This suggests that DTD promotes the formation of an SEI with lower charge-transfer resistance ($R_{ct}$), directly addressing one of the major bottlenecks at ultra-low temperatures by facilitating the Li+ desolvation and transfer process.
| Electrolyte | Capacity Retention at -50°C, 0.5C (%) | Inflection Voltage (V) |
|---|---|---|
| Baseline (No Additive) | 68.83 | 2.62 |
| Baseline + 1% DTD | 72.68 | 3.02 |
Solvent System Engineering: Introducing Fluoroethylene Carbonate (FEC)
The solvent system dictates the liquid range, viscosity, and Li+ solvation environment. Replacing part of the baseline solvent with fluoroethylene carbonate (FEC) yielded remarkable improvements (Scheme 7 vs. Scheme 1). The -50°C capacity retention jumped to 76.11%. The mechanism is multifaceted. First, the strong electron-withdrawing -F group on FEC reduces the electron density on its carbonyl oxygen, weakening its coordination with Li+. This effectively lowers the desolvation energy barrier, a rate-limiting step at low temperature, thereby reducing $R_{ct}$. This can be conceptually represented by a lower energy state for the desolvated Li+:
$$ \text{Li(Solvent)}_n^+ \rightarrow \text{Li}^+_{\text{(interface)}} + n\text{Solvent} \quad \Delta G_{\text{desolv}} (\text{with FEC}) < \Delta G_{\text{desolv}} (\text{without FEC}) $$
Second, FEC readily decomposes to form a LiF-rich SEI. LiF has high interfacial energy and good mechanical properties, which helps suppress lithium dendrite growth and creates a more stable interface. Third, the -F group improves the wettability of the electrolyte on the electrodes, ensuring better interfacial contact. Notably, the FEC-containing electrolyte also enhanced room-temperature cycle life, demonstrating its multifunctional benefit for the li ion battery.
Integrated Optimization and Performance Synergy
Having identified the individual benefits of a hard carbon anode, LiFSI co-salt, DTD additive, and FEC co-solvent, we integrated these elements into a holistic, optimized design (Scheme 8). The results, when compared to the baseline (Scheme 1), demonstrate a powerful synergistic effect. The comprehensive property improvements are evident in the bulk electrolyte conductivity (Table 6) and the final cell performance (Figure 7 data representation).
| Electrolyte Scheme | Ionic Conductivity (mS/cm) at Temperature (°C) | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| -50 | -40 | -30 | -20 | -10 | 0 | 10 | 20 | 30 | 40 | 50 | |
| 1 (Baseline) | 0.34 | 0.74 | 1.33 | 2.17 | 2.94 | 3.92 | 5.78 | 6.66 | 7.69 | 8.48 | 9.23 |
| 8 (Optimized) | 1.01 | 1.62 | 2.24 | 3.10 | 3.87 | 4.97 | 5.90 | 6.83 | 8.01 | 8.83 | 9.73 |
The optimized electrolyte’s conductivity is approximately three times higher than the baseline at -50°C. This dramatic improvement directly translates to superior cell performance. The integrated li ion battery (Scheme 8) demonstrated robust charge acceptance at -40°C (0.2C) and highly efficient discharge at -50°C (0.5C), achieving a capacity retention significantly above 75% in the latter test, compared to ~69% for the baseline. The discharge voltage plateau was also notably higher, indicating lower overall polarization. This integrated approach successfully addresses the multifaceted challenges: the hard carbon anode provides kinetically favorable Li+ storage sites; the FEC/LiFSI/DTD electrolyte ensures high bulk conductivity, low desolvation energy, and a stable, low-impedance SEI.
Conclusions and Future Perspectives
This investigation underscores that enabling a li ion battery for ultra-low temperature service is not a single-material breakthrough but a system-level integration challenge. We have demonstrated that the anode material is the critical enabler for low-temperature charging, with hard carbon’s disordered structure proving far superior to graphite. Concurrently, the electrolyte formulation holds the key to unlocking discharge performance, where enhancing Li+ transport and lowering interfacial resistance are paramount. The most significant performance gains were achieved by meticulously tailoring the Li+ solvation structure through advanced salts like LiFSI and solvents like FEC, and by refining the SEI chemistry with additives like DTD.
The optimized li ion battery developed here, capable of operating at -50°C for discharge and -40°C for charging, represents a substantial advancement. It provides a viable technical pathway for powering equipment in extreme environments, from terrestrial polar regions to the surface of the Moon. Looking forward, research must delve even deeper into the molecular-scale understanding of the electrolyte structure at cryogenic temperatures and the precise composition of the SEI formed under these conditions. Further exploration of novel salts, concentrated electrolytes, or localized high-concentration concepts could push the boundaries even further. The continuous evolution of the li ion battery, conquering one harsh environment after another, remains essential for powering the next era of human and robotic exploration.