CATL's SECRET to “Zero” Degradation Batteries: Unveiling the Potential of LFO Cathode Additives

CATL and Yutong's collaboration has yielded LFP batteries boasting a lifespan of up to 15 years, with "zero" degradation for the first 1000 cycles. Similarly, CATL's new 6.25 MWh ESS boasts a 30 % increase in energy density with a 20 % smaller footprint and claims “zero” degradation for the first 5 years. But how do they achieve this seemingly impossible feat? This article delves into one of the potential secret behind this breakthrough: cathode additives.

Understanding Battery Degradation: The Loss of Lithium Inventory

At the heart of a lithium-ion battery lies a reservoir of lithium ions that shuttle between the anode and cathode during charge and discharge cycles. With each cycle, a small number of Li ions become trapped or lost, akin to a bucket with a slow leak. This phenomenon, known as the loss of lithium inventory (LLI), gradually diminishes the battery's capacity.

The Suspect: Sacrificial Lambs – Cathode Additives

They possess several key characteristics.

• The lithium source should yield a higher capacity per unit mass than the active material, compensating for the substitution of some active material with the cathode additive and minimizing the penalty from residual inactive mass.
• They minimize inactive material content, as excess material adds weight without contributing to energy output.
• Compatibility with the current cell manufacturing process is essential.
• The N/P ratios are matched to prevent excess lithium plating on the graphite anode.
• If a significant amount of gas is released, it should occur during the cell formation stage.
• These additives require stability, compatibility with other battery components, and cost-effective production.

Li5FeO4 (LFO): A Promising Candidate

Li5FeO4 (LFO), emerges as a promising candidate for the cathode additive role. This lithium iron oxide which has an antifluorite structure boasts a remarkably high theoretical capacity 867 mAh/g, translating to over 700 mAh/g in practical use for storing lithium ions. However, unlike the main cathode material, LFO releases its lithium ions permanently, transforming into an inactive material, amorphous LiFeO2 upon fulfilling its purpose. This trade-off is acceptable because LFO packs a significant punch, releasing four times more lithium ions than the standard LFP cathode material used in these batteries. LFO is a powder which has very similar particle size to LFP and other physical properties. It can be easily mixed with LFP cathode powder in the cathode slurry result in uniform coating. Additionally, LFO's low surface area minimizes unwanted side reactions within the cell. However, it possesses extremely high sensitivity to moisture, necessitating a strictly dry handling environment and releases O2 gas during the first charging process.

How LFO Works:

• During the first formation cycle upon charging, LFO releases lithium ions in two stages (around 3.5 V and 4.0 V vs Li/Li+).

o    Li5FeO4 ---> Li3FeO3.5 + 2Li+ + 0.25O2 (at 3.5 V)

o    Li3FeO3.5 ---> LiFeO2 + 2Li+ + 0.75O2 (at 4.0 V)

• This released lithium compensates for the initial lithium loss in the first cycle, a significant factor affecting LFP capacity and cycle life.
• LFO also promotes the formation of a stable solid electrolyte interphase (SEI) in the initial cycles, reducing further lithium loss and improving cycle life.

Benefits of LFO: Extended Lifespan and Improved Efficiency

The incorporation of LFO offers several advantages for battery life.

The SEI formation process consumes some precious lithium ions during the initial cycles. For LFP, typically the formation cycle efficiency is around 85 – 90 % which resulting underutilization of the LFP cathodes (135 – 145 mAh/g) With LFO in the mix, the cell achieves a higher coulombic efficiency above 98 % during the formation cycle. This translates to a more stable SEI layer, increased lithium inventory, reduced stress on battery components, and ultimately, a longer lifespan.

LFP batteries experience the most significant lithium inventory loss during the initial cycles, where the capacity decay is dominant in the early cycles before reaching a steady state. By introducing LFO, the battery compensates for this initial loss, enabling the full utilization of the cathode material's potential (160 mAh/g).

While we lack direct confirmation, circumstantial evidence points towards LFO:

• REPT Battero's similar findings with LFO in 280 Ah ESS cells align with CATL's zero degradation claim for 1000 cycles

• Last year, Dynanonic, CATL's main supplier of LFP cathode powder, initiated commercial production of LFO under the brand "INFINILI" boasting a capacity of 20,000 tonnes per year. This suggests significant demand, potentially from CATL. Dynanonic stated that incorporating 1.5 % to 2 % of cathode additives in the battery system could boost energy density by up to 5 %, while an addition of 3 % to 4 % could nearly double the cycling life

Beyond LFO: A Multifaceted Approach

While LFO is a strong contender, it might not be the sole contributor responsible for CATL's success. The company has also hinted at the utilization of a specialized self-assembled electrolyte that generates a biomimetic SEI. Additionally, refinements to other battery components, cell manufacturing and the BMS likely play a role. This synergy between various advancements is likely the key to achieving the remarkable zero-degradation claim.

The Trade-Off: Energy Density vs. Lifespan

The use of LFO presents a potential trade-off. The presence of inactive material raises questions about the increase in energy density. Due to the immediate impact of LFO on enhancing cell capacity, setting a nominal capacity becomes challenging for cell manufacturers. Consequently, they opt to base the nominal capacity solely on the capacity derived from active material. When considering the total mass, a portion consists of inactive material compared to the base case, potentially resulting in a slightly lower overall energy density. Therefore, the increase in energy density is somewhat debatable; however, the longevity of the cell is evident from the additional lithium inventory and the early improvement in coulombic efficiency.

LFO's Potential Beyond LFP Batteries

The potential applications of LFO extend beyond LFP batteries. The industry is exploring its use with other cathode materials (NMC and LCO), particularly when paired with high-capacity anode materials like silicon, which suffer from high initial lithium loss. LFO has the potential to serve as a more affordable alternative to current pre-lithiation strategies for these next-generation batteries.

References

1.    Innovazone Commissions 20,000 Tons of Cathode Additive Project in Phase I (https://dynanonic.com/ennewsinfo.aspx?id=100).

2.    Liu, X. et al. Addressing the initial lithium loss of lithium ion batteries by introducing pre-lithiation reagent Li5FeO4/C in the cathode side. Electrochim. Acta 481, (2024).

3.    Dose, W. M. et al.  Assessment of Li-Inventory in Cycled Si-Graphite Anodes Using LiFePO4 as a Diagnostic Cathode . J. Electrochem. Soc. 165, A2389–A2396 (2018).

4.    Dose, W. M. et al.  Beneficial Effect of Li5FeO4 Lithium Source for Li-Ion Batteries with a Layered NMC Cathode and Si Anode . J. Electrochem. Soc. 167, 160543 (2020).

5.    Zhan, C. et al. Enabling the high capacity of lithium-rich anti-fluorite lithium iron oxide by simultaneous anionic and cationic redox. Nat. Energy 2, 963–971 (2017).