Accurately estimating a battery’s state of charge (SOC) is paramount for effective battery management systems, directly impacting performance, safety, and longevity in everything from electric vehicles to grid-scale energy storage. The open circuit voltage (OCV) method stands as a cornerstone for SOC estimation due to its simplicity and precision. However, for lithium iron phosphate/graphite (LFP/Gr) batteries, precisely defining the SOC-OCV curve and understanding the subtle factors that influence it—remains a complex challenge.
While significant research focuses on calibrating these curves, there’s a notable gap in understanding how fundamental elements like the specific properties of active materials, the effects of capacity decay, the implications of silicon doping, and the process of pre-lithiation alter the OCV curve. Moreover, the distinctive “voltage step” observed near 60% SOC in LFP/Gr batteries, and how it relates to the individual contributions of LFP and graphite, lacks comprehensive explanation in existing literature.
This article aims to address these critical gaps. Drawing upon a wealth of accumulated experimental data, we provide a detailed summary of how various factors influence the SOC-OCV curve of LFP/Gr batteries. Our investigation spans the intrinsic characteristics of both lithium iron phosphate and graphite active materials, the impact of battery form factor (square vs. soft pack), the direction of SOC adjustment (charging vs. discharging), the crucial role of standing time for depolarization, and the consequences of battery capacity attenuation (due to both storage and cycling). Furthermore, we explore how advanced modifications like negative electrode silicon doping and pre-lithiation alter the OCV curve, offering crucial insights for future battery design and management.
1. Experimental Protocol: Building and Testing the Cells
Our systematic investigation into the SOC-OCV curve required carefully selected experimental batteries and a rigorously standardized testing methodology.
1.1 Experimental Battery Specifications: A Tale of Two Form Factors
The study incorporated two common and distinct types of lithium iron phosphate batteries to ensure broad applicability of our findings:
- Polymer Soft-Pack Batteries: These compact cells measured 3.0mm x 62mm x 85mm and possessed a nominal capacity of approximately 2.2 Ah. These are typical for consumer electronics and smaller-scale applications.
- Square Aluminum Shell Power Batteries: Representing high-power applications, these significantly larger cells measured 60mm x 220mm x 112mm and boasted a substantial capacity of 172 Ah. Such batteries are commonly found in electric vehicles and large energy storage systems.
This dual-battery approach allowed us to compare OCV behavior across different scales and use cases, providing a more comprehensive understanding.
1.2 Performance Testing Protocol: Ensuring Accurate OCV Readings
Determining the precise SOC-OCV curve for LFP/Gr batteries demanded a methodical approach to minimize external influences and ensure equilibrium readings. The current used for adjusting the battery’s SOC was consistently set at a moderate 0.33C. This rate is carefully chosen to reduce dynamic polarization effects that could otherwise obscure true OCV values.
To capture the potential for hysteresis in the OCV response, we differentiated our SOC adjustment processes:
- Charging SOC-OCV Curve: Measurements were taken as the battery was incrementally charged to specific SOC points.
- Discharging SOC-OCV Curve: Measurements were recorded as the battery was incrementally discharged to specific SOC points.
A critical step in obtaining accurate OCV readings is allowing the battery sufficient time to depolarize. Unless specifically noted otherwise, after each adjustment of the battery’s SOC, it was left to stand for a full 4 hours. This standing period is vital; it permits the electrochemical reactions within the battery to reach a near-equilibrium state, ensuring that the measured OCV truly reflects the thermodynamic potential at that particular SOC. This rigorous method helps to decouple the intrinsic material properties from transient voltage drops.
2. Results and Analysis: Unveiling the Curves Secrets
Our extensive experimental data provides profound insights into the electrochemical behavior of LFP/Gr batteries, highlighting the intricate relationship between material properties, manufacturing variations, and the resulting SOC-OCV curve.
2.1 Decoding the LFP/Gr Battery SOC-OCV Curve
The SOC-OCV curve of an LFP/Gr battery is a macroscopic reflection of the coupled lithium ion insertion and extraction processes occurring simultaneously at both the positive (LFP) and negative (graphite) electrodes. As the SOC increases (during charging), the LFP positive electrode gradually de-lithiates, transforming from LiFePO4 into the iron phosphate (FePO4) phase. Concurrently, the graphite negative electrode gradually intercalates lithium ions, transitioning through various lithiated graphite phases (LiCx). The distinct shape of the SOC-OCV curve, particularly its platforms and steps, is thus a direct manifestation of these complex phase transitions within the active materials.
The OCV of the LFP/Gr soft-pack battery generally increases with SOC, but not uniformly. Its SOC-OCV curve can be distinctly divided into five characteristic intervals:
- 0-32% SOC (Initial Steep Rise): The OCV changes significantly in this region, increasing by 559 mV. This substantial change accounts for 89.44% of the total OCV change across the entire 0-100% SOC range. This initial steep slope is crucial for precise low-SOC estimation.
- 32-55% SOC (First Voltage Platform): Here, the OCV enters a relatively flat plateau, increasing by only 4 mV, which accounts for a mere 0.64% of the total OCV change. This flatness often complicates SOC estimation in this range.
- 55%-65% SOC (The “Voltage Step”): This is a critical region characterized by a distinct “step” change in OCV, increasing sharply by 36 mV. This accounts for 5.76% of the total OCV change. Understanding the origin of this step is paramount for accurate SOC estimation.
- 65%-95% SOC (Second Voltage Platform): Similar to the first, this region exhibits another relatively flat platform, with OCV increasing by only 5 mV, accounting for 0.80% of the total change.
- 95%-100% SOC (Final Steep Rise): In this high-SOC range, the OCV again shows a significant increase, rising by 21 mV, representing 3.36% of the total OCV change. This final steep slope is valuable for high-SOC estimation.
2.2 Influence of Active Materials
The intrinsic properties of the active materials, even subtle differences between manufacturers, can significantly influence the SOC-OCV curve. We examined four types of lithium iron phosphate (LFP-1, LFP-2, LFP-3, and LFP-4) with varying physical properties (detailed in Table 2) that impacted their capacities (2.11, 2.02, 2.07, and 2.12 Ah, respectively).
Interestingly, while the overall shape of the OCV curves for these four LFP materials appears largely similar, suggesting that the OCV is fundamentally linked to the intrinsic characteristics of LFP materials rather than specific manufacturers, a closer inspection reveals crucial differences. Zooming into the 50% to 70% SOC range where the prominent OCV step occurs (Figure 4), we observed a distinct shift in the position of this step. From left to right along the voltage axis, the sequence of the OCV steps corresponded to the battery capacities: LFP-4 (2.12Ah) → LFP-1 (2.11Ah) → LFP-3 (2.07Ah) → LFP-2 (2.02Ah). This indicates that as the gram capacity of the lithium iron phosphate active material decreases, the SOC-OCV curve shifts to the right. This subtle variation highlights that even minor differences in material parameters from various manufacturers lead to distinct lithium deintercalation characteristics and gram capacities.
The negative electrode graphite active material also plays a significant role in shaping the battery’s overall capacity and, consequently, its SOC-OCV curve. By comparing the discharge SOC-OCV curves of soft-pack batteries where graphite was the single variable (Gr-1, Gr-2, Gr-3, and Gr-4), we again observed a noticeable effect on the OCV curve within the 50% to 70% SOC range. Similar to the LFP materials, the position of the OCV step varied with the graphite type. From left to right, the sequence was Gr-2 → Gr-4 → Gr-3 → Gr-1, which directly correlated with their respective battery capacities: Gr-2 (2.21Ah) → Gr-4 (2.20Ah) → Gr-3 (2.19Ah) → Gr-1 (2.11Ah).
The battery’s total capacity fundamentally reflects the amount of lithium ions that can be reversibly released by the positive LFP material and intercalated by the negative graphite material. Therefore, variations in the phase states of these active materials at a given SOC will inherently alter the potentials of the positive and negative electrodes, leading to different overall OCVs. Importantly, our observations show that regardless of the specific LFP or graphite material used, the characteristic voltage step in the 50% to 70% SOC range persists. This strongly suggests that this voltage step is an intrinsic characteristic of the LFP/Gr battery system, arising from the coupled phase transformations within this particular electrochemistry, rather than a peculiarity of a specific material batch.