With the global energy structure rapidly transitioning towards low-carbon alternatives, the demand for efficient and reliable energy storage solutions has never been greater. Energy storage lithium batteries, particularly lithium iron phosphate (LiFePO₄ or LFP) cells, have emerged as a cornerstone technology due to their inherent safety, long cycle life, and cost-effectiveness. These batteries are extensively deployed in various applications, from grid-scale energy storage systems to commercial and industrial facilities, where they play a pivotal role in stabilizing power supply, integrating renewable energy sources, and enhancing grid resilience. However, the operational performance of energy storage lithium batteries is profoundly influenced by environmental conditions and loading profiles, specifically temperature and charge-discharge rates. Understanding these influences is critical for optimizing system design, ensuring safety, and maximizing economic returns.
In this study, we focus on the performance evaluation of 314Ah LFP cells designed for electrical energy storage applications. These large-format cells are increasingly favored in the industry for their higher energy density and reduced system integration costs compared to smaller counterparts. We conducted a series of experiments under controlled conditions to investigate how temperature and C-rate variations affect key performance metrics such as energy efficiency, voltage characteristics, thermal behavior, and capacity retention. The tests were performed in accordance with the GB/T 36276-2023 standard, which provides a rigorous framework for assessing the initial and operational characteristics of energy storage lithium batteries. Our objective is to provide empirical data and analytical insights that can guide the development of more robust and efficient energy storage systems, particularly in challenging environments and high-power applications.
The fundamental operation of an energy storage lithium battery involves the reversible movement of lithium ions between the cathode and anode during charging and discharging cycles. For LFP cells, the cathode material is LiFePO₄, which offers a stable olivine structure, while the anode typically consists of graphite. The electrochemical reactions can be summarized as follows:
Positive electrode reaction: $$\text{LiFePO}_4 \leftrightarrow \text{Li}_{1-x}\text{FePO}_4 + x\text{Li}^+ + x\text{e}^-$$
Negative electrode reaction: $$\text{C}_6 + x\text{Li}^+ + x\text{e}^- \leftrightarrow \text{Li}_x\text{C}_6$$
Overall reaction: $$\text{LiFePO}_4 + 6\text{C} \leftrightarrow \text{Li}_{1-x}\text{FePO}_4 + \text{Li}_x\text{C}_6$$
During charging, lithium ions de-intercalate from the LiFePO₄ cathode, migrate through the electrolyte, and intercalate into the graphite anode, with electrons flowing through the external circuit. Discharging reverses this process, releasing stored energy. The efficiency and stability of these reactions are highly dependent on operational parameters, which we explore in detail through our experimental findings.
Experimental Methodology
We selected commercial 314Ah prismatic LFP cells with a nominal voltage of 3.2V for this investigation. The charging cutoff voltage was set at 3.65V, and the discharging cutoff at 2.5V, consistent with typical specifications for energy storage lithium batteries. To ensure reproducibility, we used multiple cells from the same production batch with closely matched electrical characteristics. The tests were conducted using a precision battery cycler, and environmental conditions were controlled using a thermal chamber. High-accuracy temperature sensors (±0.5°C) were attached to the center of the cell’s large surface to monitor surface temperature variations in real-time.
The experimental design encompassed three primary test series:
Temperature Tests: Cells were subjected to initial charge-discharge performance tests at three distinct ambient temperatures: 5°C, 25°C, and 45°C. The procedure followed Section 6.4.1 of GB/T 36276-2023, where each cell underwent initialization discharge followed by a 1P constant power charge-discharge cycle until the voltage limits were reached.
C-rate Tests: At a controlled ambient temperature of 25±2°C, cells were evaluated at different constant power rates: 1P, 2P, and 4P. These tests adhered to Sections 6.4.3 and 6.7.1.3 of the standard, assessing rate capability and overload performance. The power levels were defined based on the cell’s rated capacity, with 1P corresponding to approximately 1005W for a 314Ah cell at 3.2V.
Combined Rate-Temperature Tests: A subset of cells was tested at a 2P rate under two slightly different ambient temperatures (25°C and 26°C) to examine the sensitivity of performance to minor temperature fluctuations, particularly in relation to the standard’s efficiency requirements.
Throughout all tests, we recorded voltage, current, energy input/output, time, and surface temperature at high sampling rates to capture dynamic behaviors. The energy efficiency (η) was calculated as: $$\eta = \frac{E_{\text{discharge}}}{E_{\text{charge}}} \times 100\%$$ where \(E_{\text{charge}}\) and \(E_{\text{discharge}}\) represent the total energy during charging and discharging, respectively.
Impact of Temperature on Battery Performance
Temperature is a critical factor influencing the electrochemical behavior of energy storage lithium batteries. Our experiments revealed substantial variations in energy efficiency, voltage profiles, and thermal characteristics across the tested temperature range.
The data from the temperature-dependent tests are summarized in Table 1. At 25°C, the energy storage lithium battery exhibited an initial energy efficiency of approximately 95%. When the temperature increased to 45°C, efficiency improved to 97.5%, accompanied by higher charge and discharge energies. In contrast, at 5°C, efficiency dropped significantly to 82.4%, with a more pronounced reduction in discharge energy, highlighting the limitations of LFP chemistry in cold environments.
| Parameter | Unit | 5°C | 25°C | 45°C |
|---|---|---|---|---|
| Charge Energy | Wh | 1005.1 | 1089.4 | 1101.7 |
| Discharge Energy | Wh | 828.3 | 1031.5 | 1074.2 |
| Energy Efficiency | % | 82.4 | 94.7 | 97.5 |
| Max Charge Temperature | °C | 8.1 | 29.8 | 49.4 |
| Max Discharge Temperature | °C | 11.3 | 30.8 | 50.8 |
| Charge Temperature Rise | °C | 3.9 | 1.5 | 2.6 |
| Discharge Temperature Rise | °C | 4.9 | 2.8 | 2.5 |
The voltage-energy curves during charge and discharge cycles provide further insights. At 25°C and 45°C, the charge voltage plateaus were situated in the 3.35–3.45V range, with the higher temperature extending the plateau duration and increasing the total charge energy. At 5°C, the charge plateau shifted upward to 3.4–3.5V and shortened, resulting in reduced energy input. Similarly, discharge plateaus were highest at 45°C (3.3–3.2V), moderate at 25°C (3.25–3.15V), and lowest at 5°C (3.2–3.1V), directly impacting the usable energy output.
Thermal analysis showed that temperature rises during operation were modest at 25°C and 45°C, staying below 3°C. However, at 5°C, the temperature rise was more pronounced, reaching 3.9°C during charging and 4.9°C during discharging. Interestingly, at 25°C and 5°C, discharge processes generated higher temperature rises than charging, but this trend reversed at 45°C, where charging produced slightly more heat. These observations can be attributed to the combined effects of reversible reaction heat and irreversible polarization heat.
The heat generation in an energy storage lithium battery primarily stems from two sources: reaction heat and polarization heat. Reaction heat is associated with the entropy changes during lithium intercalation and de-intercalation, while polarization heat arises from overpotentials due to ohmic resistance, charge transfer limitations, and concentration gradients.
Ohmic polarization is described by: $$\Delta E_{\text{ohmic}} = I R$$ where \(I\) is the current and \(R\) is the internal resistance.
Electrochemical polarization follows the Butler-Volmer equation: $$i = i_0 \left( \exp \left[ \frac{\alpha_a F \eta}{RT} \right] – \exp \left[ -\frac{\alpha_c F \eta}{RT} \right] \right)$$ where \(i\) is the current density, \(i_0\) is the exchange current density, \(\alpha_a\) and \(\alpha_c\) are the anodic and cathodic charge transfer coefficients, \(F\) is Faraday’s constant, \(\eta\) is the activation overpotential, \(R\) is the gas constant, and \(T\) is the absolute temperature.
Concentration polarization becomes significant at high currents due to limited ion diffusion rates in the electrolyte and active materials.
At lower temperatures, the increase in electrolyte viscosity and charge transfer resistance amplifies polarization effects, necessitating higher voltages to drive reactions and leading to greater energy losses as heat. This explains the elevated temperature rises observed at 5°C. The asymmetry in charge-discharge temperature rises stems from differences in the kinetics of lithium intercalation and de-intercalation in graphite; de-intercalation during discharge typically faces higher energy barriers, especially at low temperatures, resulting in more heat generation.
To delve deeper into the temperature effects, we employed incremental capacity analysis (ICA), which differentiates the voltage-capacity curves to highlight phase transition behaviors. The IC curves, derived from \(dQ/dV\) versus voltage, show distinct peaks corresponding to major electrochemical reactions. At elevated temperatures, these peaks shift to lower voltages during charging and higher voltages during discharging, indicating reduced polarization and improved reaction kinetics. At 5°C, the peaks broaden and diminish in amplitude, reflecting increased polarization and hindered lithium diffusion, which aligns with the poor low-temperature performance of LFP-based energy storage lithium batteries.
Impact of C-rate on Battery Performance
The rate at which an energy storage lithium battery is charged or discharged significantly influences its efficiency, thermal behavior, and longevity. Our tests at 1P, 2P, and 4P constant power rates demonstrated that LFP cells maintain stable voltage plateaus and consistent charge energies across different rates, underscoring their suitability for various application scenarios.
Table 2 compiles the key performance metrics from the C-rate tests. As the rate increased from 1P to 4P, the energy efficiency gradually declined, and temperature rises became more substantial. At 1P, the temperature rise during discharge was slightly higher than during charge, but both remained below 3°C. At 4P, however, the charge process generated a temperature rise exceeding 9°C, indicating intensified thermal effects at high power levels.
| Parameter | Unit | 1P | 2P | 4P |
|---|---|---|---|---|
| Charge Energy | Wh | 1089.4 | 1109.4 | 1115.8 |
| Discharge Energy | Wh | 1031.5 | 993.2 | 980.1 |
| Energy Efficiency | % | 94.7 | 89.5 | 87.8 |
| Max Charge Temperature | °C | 29.8 | 34.2 | 38.5 |
| Max Discharge Temperature | °C | 30.8 | 35.1 | 37.0 |
| Charge Temperature Rise | °C | 1.5 | 6.2 | 9.3 |
| Discharge Temperature Rise | °C | 2.8 | 5.9 | 8.5 |
The voltage-energy curves reveal that higher C-rates cause an upward shift in the charge voltage plateau and a downward shift in the discharge plateau, primarily due to increased ohmic losses and polarization. At 4P, the discharge voltage drops more rapidly, curtailing the effective discharge period and reducing the delivered energy. This behavior is characteristic of energy storage lithium batteries under high-current conditions, where ion transport limitations become predominant.
The IC curves further elucidate the rate-dependent phenomena. As the C-rate increases, the charge peaks in the \(dQ/dV\) plots shift to higher voltages and broaden, indicating heightened polarization. During discharge, the peaks overlap and lose definition, signifying that the electrochemical reactions are occurring under severe kinetic constraints. The cumulative effect of ohmic, electrochemical, and concentration polarizations at high rates leads to greater energy dissipation as heat, which is consistent with the observed temperature rises.
At low rates (e.g., 1P), the intercalation of lithium into graphite during charging proceeds with relatively favorable kinetics, whereas de-intercalation during discharge encounters more resistance, explaining the higher discharge temperature rise. As rates increase, ohmic heating (\(I^2R\)) dominates, and side reactions such as electrolyte oxidation at high charge voltages contribute additional heat, making charge cycles thermally more demanding at 4P. This has important implications for the thermal management of energy storage lithium batteries in high-power applications, where charging phases may require more aggressive cooling strategies.
Combined Effects of Rate and Temperature
To explore the interplay between C-rate and temperature, we conducted additional tests on two cells at a 2P rate under ambient temperatures of 25°C and 26°C. The results, detailed in Table 3, highlight the sensitivity of energy efficiency to minor temperature variations, especially in the context of standard compliance.
| Parameter | Unit | Cell #1 at 25°C | Cell #2 at 25°C | Cell #1 at 26°C | Cell #2 at 26°C |
|---|---|---|---|---|---|
| Ambient Temperature | °C | 25.00 | 25.00 | 26.00 | 26.00 |
| 2P Charge Energy | Wh | 1109.42 | 1105.39 | 1116.60 | 1116.27 |
| 2P Discharge Energy | Wh | 993.21 | 986.61 | 1014.60 | 1016.10 |
| Energy Efficiency | % | 89.52 | 89.25 | 90.87 | 91.03 |
| Average Charge Temperature | °C | 28.16 | 25.93 | 29.25 | 29.18 |
| Average Discharge Temperature | °C | 28.19 | 26.04 | 29.85 | 30.13 |
At 25°C, the energy efficiency for both cells was below the 90% threshold specified in GB/T 36276-2023 for 2P rate cycles. When the ambient temperature increased by just 1°C to 26°C, efficiency improved to over 90%, meeting the standard requirement. This improvement was driven by a noticeable extension of the discharge voltage plateau, which increased the discharge energy by approximately 2-3%. The underlying mechanism can be explained by the Arrhenius equation: $$k = A \cdot e^{-E_a / (RT)}$$ where \(k\) is the rate constant, \(A\) is the pre-exponential factor, \(E_a\) is the activation energy, \(R\) is the gas constant, and \(T\) is temperature. A slight temperature rise reduces the activation energy barrier for charge transfer reactions, enhancing ion mobility and reducing polarization losses. This demonstrates that even small temperature adjustments can have a measurable impact on the performance of energy storage lithium batteries, particularly under high-rate conditions.
Conclusion
Our comprehensive investigation into the performance of 314Ah LFP cells under varied temperatures and C-rates yields several key conclusions that are directly relevant to the design and operation of energy storage lithium battery systems.
First, temperature exerts a profound influence on the efficiency and behavior of energy storage lithium batteries. Higher temperatures (e.g., 45°C) reduce internal resistance and polarization, enabling higher energy efficiency and more stable voltage plateaus. In contrast, low temperatures (e.g., 5°C) severely impair performance by increasing electrolyte viscosity and charge transfer resistance, leading to significant efficiency losses and elevated temperature rises. For energy storage systems deployed in cold climates, active heating solutions or selecting batteries with enhanced low-temperature capabilities are essential to mitigate these effects.
Second, LFP cells demonstrate excellent stability across a range of C-rates, making them versatile for both energy-intensive and power-intensive applications. At low rates, the thermal management burden is minimal, with temperature rises under 3°C. However, as rates increase to 4P, polarization and ohmic heating intensify, particularly during charging, resulting in temperature rises exceeding 9°C. This asymmetry between charge and discharge thermal behavior necessitates tailored cooling strategies, especially in high-rate applications like frequency regulation, where charging cycles may demand more robust thermal control.
Third, the interplay between rate and temperature highlights the importance of precise environmental control. Even a 1°C increase in ambient temperature can boost energy efficiency above critical thresholds, as observed in the 2P rate tests. This sensitivity underscores the need for advanced battery management systems that dynamically adjust operating parameters based on real-time conditions to optimize performance and safety.
In summary, energy storage lithium batteries based on LFP chemistry offer a compelling combination of safety, longevity, and cost-effectiveness, but their performance is intimately tied to operational conditions. By understanding and addressing the impacts of temperature and C-rate, system designers can enhance efficiency, extend lifespan, and ensure reliable operation across diverse scenarios. Future work should focus on developing adaptive thermal management algorithms and exploring material modifications to further improve the performance boundaries of energy storage lithium batteries in extreme environments.