My doctoral research focuses on the cathode pre-metallization process for high energy density energy storage battery and capacitor systems. The development of renewable energy sources such as wind and solar power has been accelerating globally, yet their intermittent and unstable nature poses significant challenges for grid integration. High-performance electrochemical energy storage devices are essential for achieving effective “peak shaving” of renewable energy, which has driven my interest in investigating advanced pre-metallization strategies for high energy density energy storage battery systems.
Research Background and Significance
In my investigation of high energy density energy storage battery technologies, I recognized that both lithium-ion capacitors (LICs) and sodium-ion batteries (NIBs) face a common critical challenge: the irreversible loss of metal ions during the first charge-discharge cycle. This loss primarily occurs due to the formation of the solid electrolyte interface (SEI) layer on anode surfaces, which consumes significant amounts of Li⁺ or Na⁺ ions. The cathode pre-metallization process, which introduces sacrificial lithium/sodium-rich materials into the cathode, has emerged as one of the most effective strategies to compensate for these irreversible losses. My research aims to develop optimized pre-metallization processes that can significantly enhance the energy density and cycling stability of high energy density energy storage battery devices.
The energy density E of an energy storage device can be expressed as:
$$E = \int_{t_1}^{t_2} \frac{I\Delta V}{3.6m} dt$$
Where I is the current, ΔV is the voltage window, m is the total active mass, and t is the discharge time. Similarly, the power density P is given by:
$$P = \frac{E}{t} = \Delta V \cdot \frac{I}{m}$$
The theoretical decomposition capacity of any sacrificial reagent can be calculated using Faraday’s law:
$$C = \frac{96500n}{3.6M} \text{ mAh g}^{-1}$$
Where n is the number of electrons transferred per mole of material, and M is the molecular weight of the reactant.
Lithium-Ion Capacitors and Their Challenges
In my first research direction, I focused on high energy density energy storage battery capacitors based on lithium-ion systems. The lithium-ion capacitor combines the high energy density of lithium-ion batteries with the high power density of supercapacitors. The dual-carbon LIC system, consisting of activated carbon (AC) as cathode and hard carbon (HC) as anode, offers advantages including wide voltage range, excellent physical and chemical stability, and outstanding cycling performance. However, both carbon materials lack intrinsic lithium sources, which creates a fundamental lithium deficiency problem in the energy storage battery system.
The lithium deficiency in dual-carbon LICs means that the formation of the SEI layer during the first cycle consumes Li⁺ ions from the electrolyte irreversibly. This results in severe capacity loss and reduced energy density. The pre-lithiation process is therefore essential for practical LIC implementation. In my work, I investigated cathode sacrificial lithium salts as a pre-lithiation approach due to their process compatibility with existing battery manufacturing infrastructure. These sacrificial salts decompose irreversibly during the first charge, releasing lithium ions that can compensate for the SEI formation losses.
Selection of Li₂NiO₂ as Pre-lithiation Reagent
I selected Li₂NiO₂ (LNO) as the pre-lithiation reagent for my LIC study. Through solid-state synthesis, I reacted equimolar amounts of Li₂O and NiO at 650°C under argon atmosphere. The XRD analysis confirmed the successful synthesis of Li₂NiO₂ with characteristic peaks, while SEM imaging revealed irregular particles of approximately 60 μm diameter. The electrochemical evaluation showed that LNO exhibited an initial charge capacity of 312 mAh g⁻¹ and discharge capacity of 105 mAh g⁻¹ at 25 mA g⁻¹, resulting in an irreversible capacity of 207 mAh g⁻¹. This significant irreversibility makes LNO an excellent lithium supplement candidate. The cyclic voltammetry analysis showed a broad oxidation peak around 3.8 V, corresponding to lithium extraction from the lattice.
However, I identified a critical limitation: compared to other sacrificial reagents, LNO’s lithium supply capacity is moderate, requiring relatively high addition amounts in LIC applications (typically >30%). To address this issue, I proposed a novel synergistic strategy combining LNO with electrolyte additives.
Role of LiDFOB Electrolyte Additive
My key innovation was the introduction of lithium difluoro(oxalato)borate (LiDFOB) as an electrolyte additive during the pre-lithiation process. The decomposition voltage of common electrolyte solvents (EC, DMC, EMC) is typically around 4.3 V vs. Li/Li⁺. During the first charge process, electrolyte decomposition at the anode contributes to SEI formation. LiDFOB decomposes preferentially to form a stable, LiF-rich SEI layer on the hard carbon anode surface, which is more effective at preventing further electrolyte decomposition compared to the SEI formed in conventional electrolytes.
The chemical reactions during SEI formation with LiDFOB can be represented as:
$$\text{LiDFOB} + e^- \rightarrow \text{LiF} + \text{oxalate-boron species}$$
$$\text{Li}_2\text{NiO}_2 \rightarrow \text{LiNiO}_2 + \text{Li}^+ + e^-$$
My electrochemical testing revealed that the combination of 20% LNO in the cathode and 2% LiDFOB in the electrolyte produced the optimal performance. Table 1 summarizes the energy efficiency of full capacitors with different LiDFOB additions.
**Table 1: Energy efficiency of AC-20%LNO//HC full capacitors with different LiDFOB contents**
| LiDFOB Content (%) | Energy Efficiency (%) | Optimal Performance |
|——————-|———————-|——————-|
| 1 | 73 | No |
| 2 | 80 | Yes |
| 3 | 78 | No |
| 5 | 75 | No |
Electrochemical Performance of Pre-lithiated LIC
The synergistic effect between LNO and LiDFOB enabled the pre-lithiation of my LIC with remarkably low LNO addition (20%). The pre-lithiated full capacitor, composed entirely of commercial materials (activated carbon//hard carbon), achieved outstanding performance metrics for a high energy density energy storage battery device:
$$E = 98.53 \text{ Wh kg}^{-1} \text{ at } P = 31 \text{ W kg}^{-1}$$
$$P = 4.65 \text{ kW kg}^{-1} \text{ at } E = 19.24 \text{ Wh kg}^{-1}$$
The long-term cycling stability was also impressive. At a current density of 0.5 A g⁻¹, the full capacitor retained 75.1% of its initial capacity after 5,000 cycles. More remarkably, at the low current density of 0.01 A g⁻¹, complete capacity retention was observed over 500 cycles.
I conducted dynamic analysis using the relationship between current response and scan rate in cyclic voltammetry. The total stored charge can be separated into surface capacitive and diffusion-controlled contributions:
$$i(V) = k_1v + k_2v^{1/2}$$
By rearranging this equation:
$$\frac{i(V)}{v^{1/2}} = k_1v^{1/2} + k_2$$
Where k₁v represents the surface capacitive contribution and k₂v^(1/2) represents the diffusion-controlled (battery-type) contribution. My analysis revealed that increasing the LNO addition from 15% to 30% increased the diffusion-controlled contribution from 11.5% to 27.9%, confirming that LNO decomposition products contribute significantly to the cathode capacity.
| Additive | Addition (wt%) | Sacrificial Capacity (mAh g⁻¹) | Decomposition Potential (V) | Energy Density (Wh kg⁻¹) |
|---|---|---|---|---|
| Li₅ReO₆ | 40 | 500 | 4.3 | 65 |
| Li₂C₄O₄ | 50 | 350 | 3.8 | 58 |
| Li₃N | 10 | >1200 | 1.0 | 74.7 |
| Na₂C₂O₄ | 20 | 300 | 3.95 | 56.4 |
| Li₂DHBN | 50 | 345 | 3.5 | 50 |
| Li₂NiO₂ + LiDFOB (This work) | 20 | 213 | 3.6 | 98.53 |
Mechanism Investigation of SEI Modification
My three-electrode cell experiments provided critical insights into the pre-lithiation mechanism. In the pristine AC//HC full capacitor without pre-lithiation, severe polarization was observed during the first cycle, with the negative electrode potential rising significantly due to insufficient Li⁺ supply. The negative electrode potential ranged from 0.17 to 1.39 V during cycling. After pre-lithiation with LNO and LiDFOB, the negative electrode potential decreased to 0.05-0.67 V, approaching the ideal potential distribution for a high-performance LIC.
The X-ray photoelectron spectroscopy (XPS) analysis of the hard carbon anode after cycling revealed significant differences in the SEI composition between pre-lithiated and non-pre-lithiated cells. The F 1s spectra showed peaks at 686.1 eV and 685 eV corresponding to P-O-F species (from LiPF₆ decomposition) and LiF, respectively. In the electrolyte containing LiDFOB, the content of LixPFyOz species was significantly lower, indicating reduced LiPF₆ decomposition. The enhanced contents of alkyl lithium carbonate (ROCO₂Li) and Li₂CO₃ in the pre-lithiated cell confirmed that LNO provided sufficient Li⁺ for forming favorable SEI components while LiDFOB regulated the SEI composition.
The SEM imaging of the hard carbon anode after cycling showed that without pre-lithiation, the SEI layer was jagged and uneven, with visible detachment of hard carbon particles from the current collector. After pre-lithiation, a much more uniform and stable SEI layer was formed, greatly enhancing the structural stability of the electrode during long-term cycling.
Sodium-Ion Batteries and Pre-sodiation Process
In my second research direction, I addressed the pre-sodiation challenge in sodium-ion batteries (NIBs) for high energy density energy storage battery applications. Sodium-ion batteries have emerged as promising alternatives to lithium-ion batteries due to the abundance of sodium resources. However, several challenges specific to NIBs require attention: most cathode materials are sodium-deficient (Na content < 1 per formula unit), and the primary anode material (hard carbon) exhibits a low initial Coulombic efficiency of approximately 80%. This combination leads to severe sodium loss during the first cycle, directly impacting the energy density and cycling stability of high energy density energy storage battery devices.
Development of NaHF₂-based Pre-sodiation Reagent
The core requirement for an effective pre-sodiation process lies in developing suitable sacrificial reagents. I focused on NaHF₂, a sodium-rich salt with a theoretical decomposition capacity calculated using Faraday’s law:
$$C_{theoretical} = \frac{96500 \times 1}{3.6 \times 61.99} = 432.38 \text{ mAh g}^{-1}$$
NaHF₂ offers several advantages: high environmental stability, compatibility with N-methyl-2-pyrrolidone (NMP) solvent used in electrode preparation, and resistance to moisture and oxygen. However, my initial testing revealed that the actual decomposition capacity was merely 81.3 mAh g⁻¹, far below the theoretical value. This poor performance was attributed to the low electronic conductivity and sluggish decomposition kinetics of NaHF₂.
Table 3 provides a comparison of different pre-sodiation reagents and their characteristics.
**Table 3: Comparison of pre-sodiation reagents for sodium-ion batteries**
| Reagent | Theoretical Capacity (mAh g⁻¹) | Decomposition Potential (V) | Air Stability | NMP Compatibility |
|———|——————————-|—————————|—————|——————-|
| NaNH₂ | 683 | ~2.5 | Poor | Poor |
| NaBH₄ | 945 | ~2.0 | Poor | Poor |
| Na₂C₂O₄ | 400 | ~4.4 (needs catalysis) | Good | Good |
| Na₂S | 687 | ~3.6 | Moderate | Moderate |
| **NaHF₂ (This work)** | **432** | **3.7 (with AC)** | **Good** | **Good** |
Catalytic Effect of Activated Carbon
My solution involved utilizing high-specific-surface-area activated carbon (YP80F, specific surface area ~2300 m² g⁻¹) as both a conductive matrix and a catalyst. Through high-energy ball milling, I achieved uniform dispersion of NaHF₂ nanoparticles (50-100 nm) on the activated carbon particles. This AC+NaHF₂ composite demonstrated remarkably different decomposition behavior compared to pristine NaHF₂.
The decomposition reaction can be represented as:
$$2\text{NaHF}_2 \rightarrow 2\text{Na}^+ + \text{H}_2\uparrow + 2\text{F}^- + 0.5\text{F}_2\uparrow$$
Or more simply:
$$\text{NaHF}_2 \rightarrow \text{NaF} + \text{HF}\uparrow$$
Transmission electron microscopy observations provided striking evidence of the catalytic effect. When I focused the high-energy electron beam on the sample, NaHF₂ particles embedded within or in close contact with activated carbon decomposed rapidly, while exposed NaHF₂ particles remained unchanged. This confirmed the critical role of activated carbon in facilitating NaHF₂ decomposition through enhanced electronic conductivity and catalytic active sites.
Optimization of AC:NaHF₂ Ratio
I systematically varied the mass ratio of activated carbon to NaHF₂ to optimize the pre-sodiation performance. The results are summarized in Table 4.
**Table 4: Irreversible capacities of AC+NaHF₂ composites with different mixing ratios**
| Ratio (NaHF₂:AC) | Irreversible Capacity (mAh g⁻¹ based on total mass) | Irreversible Capacity (mAh g⁻¹ based on NaHF₂ mass) | Utilization (%) |
|——————-|—————————————————|—————————————————|—————–|
| 1:1 | 246.7 | 485 | 112.1* |
| 2:3 | 310.4 | 494 | 114.2* |
| 2:1 | 55 | 144 | 33.3 |
| Pristine NaHF₂ | – | 81.3 | 18.8 |
*Values exceeding 100% include contributions from activated carbon irreversible capacity and electrolyte decomposition.
When the ratio of NaHF₂:AC was 2:3, I obtained the optimal pre-sodiation reagent with an irreversible capacity of 310.4 mAh g⁻¹ (based on total mass) and a decomposition potential as low as 3.7 V. The XRD analysis before and after electrochemical cycling confirmed the complete decomposition of NaHF₂, as evidenced by the disappearance of characteristic diffraction peaks at 19.23°, 30.36°, and 32.41°.
Performance in Sodium-Ion Full Batteries
My pre-sodiation reagent was evaluated by incorporating 5 wt% into a Li, Mg, and Ca co-doped Na₀.₇₅Ni₀.₃Mn₀.₇O₂ (LCMNMO) cathode material. The addition of the pre-sodiation reagent increased the first-cycle irreversible capacity from 23 mAh g⁻¹ to 41 mAh g⁻¹, effectively compensating for the irreversible sodium consumption during SEI formation on the hard carbon anode.
The galvanostatic intermittent titration technique (GITT) analysis provided valuable insights into the Na⁺ diffusion kinetics:
$$D_{\text{Na}^+} = \frac{4}{\pi\tau}\left(\frac{m_BV_m}{M_BA}\right)^2\left(\frac{\Delta E_s}{\Delta E_\tau}\right)^2$$
Where τ is the pulse time, m_B is the active material mass, V_m is the molar volume, M_B is the molecular weight, A is the contact area, ΔE_s is the potential change at the end of the pulse, and ΔE_τ is the potential change during the constant current pulse.
My GITT results revealed that the Na⁺ diffusion coefficients in the pre-sodiated cathode (3.1-14.5 × 10⁻¹⁰ cm² s⁻¹) were lower than those in the pristine cathode (4.5-24.98 × 10⁻¹⁰ cm² s⁻¹). This observation suggests that the pre-sodiation reagent maintained higher sodium content in the cathode during discharge, which consequently slowed the sodiation kinetics at equivalent voltages. This finding indirectly confirms the effectiveness of my pre-sodiation strategy in preventing active sodium loss.
Full Cell Performance
I matched the pre-sodiated LCMNMO cathode with a coal-based hard carbon anode (N/P ratio ≈ 1.01) to construct a sodium-ion full battery. The coal-based hard carbon anode exhibited a discharge capacity of approximately 300 mAh g⁻¹ with an irreversible capacity of 67.6 mAh g⁻¹ during the first cycle. Without pre-sodiation, the cathode’s 23 mAh g⁻¹ irreversible capacity was insufficient to compensate for this anode loss, leading to severe active sodium depletion and rapid capacity fade.
With my 5% pre-sodiation reagent addition, the full battery demonstrated exceptional performance for a high energy density energy storage battery device:
$$E = 274.4 \text{ Wh kg}^{-1} \text{ at } 0.025 \text{ A g}^{-1}$$
And at 0.05 A g⁻¹, the capacity retention was 93.6% over 150 cycles. In contrast, the non-pre-sodiated full battery showed rapid capacity decay, dropping to only 25 mAh g⁻¹ within 30 cycles. The first-cycle discharge capacity of the pre-sodiated full battery (96.25 mAh g⁻¹) was substantially higher than that of the non-pre-sodiated battery (55.2 mAh g⁻¹).
Table 5 summarizes the performance comparison between pre-sodiated and pristine full batteries.
**Table 5: Performance comparison of LCMNMO//HC full batteries**
| Parameter | Pristine | Pre-sodiated |
|———–|———-|————–|
| First-cycle discharge capacity (mAh g⁻¹) | 55.2 | 96.25 |
| Energy density (Wh kg⁻¹) | 159.8 | 274.4 |
| Capacity retention after 150 cycles | ~45% | 93.6% |
| Cathode irreversible capacity (mAh g⁻¹) | 23 | 41 |
| Anode irreversible capacity compensation | Insufficient | Complete |
Comparative Electrochemical Analysis
The energy density improvement achieved through my pre-metallization strategies can be quantified for high energy density energy storage battery systems. In the LIC system, the pre-lithiation process enhanced the energy density from 35 Wh kg⁻¹ (pristine) to 98.53 Wh kg⁻¹, representing a 182% improvement. The energy efficiency also improved from 62.5% to 80%, primarily attributed to the reduced electrolyte decomposition and the formation of a stable SEI layer.
For the NIB system, the pre-sodiation process enhanced the energy density from 159.8 to 274.4 Wh kg⁻¹, a 71.7% improvement. The cycling stability improvement was even more profound, with the capacity retention increasing from approximately 45% to 93.6% over 150 cycles.
In my pre-sodiation study, the decomposition of NaHF₂ in the presence of activated carbon can be expressed by the following reaction equilibrium:
$$\text{NaHF}_2 \xrightarrow{\text{AC catalyst}} \text{Na}^+ + \text{F}^- + \text{HF}\uparrow$$
The activated carbon serves multiple functions: providing a three-dimensional conductive network for electron transfer, offering high surface area for uniform NaHF₂ distribution, and catalytically promoting the decomposition reaction. The TEM observation of decomposition rates varying with the contact area between NaHF₂ and activated carbon provides direct evidence for this catalytic mechanism.
The kinetic enhancement can be understood through the increased reaction rate constant:
$$k = A\exp\left(-\frac{E_a}{RT}\right)$$
The activated carbon reduces the activation energy E_a for NaHF₂ decomposition by providing catalytic active sites and improved electron pathways, thereby accelerating the reaction and enabling complete decomposition at lower overpotentials.
Kinetic Analysis of Pre-metallization Processes
In my investigation of the LIC pre-lithiation process, I employed cyclic voltammetry at various scan rates (0.1 to 2.0 mV s⁻¹) to quantify the capacitive and diffusion-controlled contributions to the total current response. The relationship between peak current and scan rate follows the power law:
$$i = av^b$$
Where the b-value indicates the charge storage mechanism: b = 0.5 corresponds to diffusion-controlled processes (battery-type behavior), while b = 1.0 corresponds to surface-controlled processes (capacitive behavior). My analysis showed that the addition of LNO increased the diffusion-controlled contribution, confirming the battery-type capacity contribution from LNO decomposition products.
The battery-type contribution from LNO decomposition products can be attributed to the following reactions during discharge:
$$\text{LiNiO}_2 + x\text{Li}^+ + xe^- \rightarrow \text{Li}_{1+x}\text{NiO}_2$$
This reversible lithium insertion into the decomposition product provides additional cathode capacity beyond the electric double-layer capacitance of activated carbon, contributing to the overall energy density enhancement of the energy storage battery.
For the NIB pre-sodiation system, my GITT analysis provided quantitative Na⁺ diffusion coefficients. The diffusion coefficient of sodium ions in the cathode material was calculated using the equation presented earlier. During the charge process, the diffusion coefficient remained above 10⁻¹⁰ cm² s⁻¹ in the 2-3.6 V range but dropped sharply below 5 × 10⁻¹⁰ cm² s⁻¹ above 4 V, which I attribute to the structural phase transition of the layered oxide material at high voltages.
Implications for High Energy Density Energy Storage Battery
My research demonstrates that the cathode pre-metallization process is a critical enabling technology for high energy density energy storage battery systems. The key findings of my work can be summarized as follows:
| System | Strategy | Key Additives | Energy Density Enhancement | Cycling Improvement |
|---|---|---|---|---|
| LIC (AC//HC) | Pre-lithiation | Li₂NiO₂ (20%) + LiDFOB (2%) | 35 → 98.53 Wh kg⁻¹ | 5000 cycles, 75.1% retention |
| NIB (LCMNMO//HC) | Pre-sodiation | AC+NaHF₂ (5%) | 159.8 → 274.4 Wh kg⁻¹ | 150 cycles, 93.6% retention |
The pre-lithiation process for LICs integrated pre-lithiation reagents with electrolyte additives to achieve efficient SEI formation while minimizing electrolyte decomposition. The LiDFOB additive played a crucial role in SEI modification by promoting the formation of a LiF-rich SEI layer that effectively suppressed further electrolyte decomposition. This synergistic approach allowed me to reduce the LNO addition to 20%, much lower than typical values reported for other sacrificial reagents in high energy density energy storage battery applications.
The electrochemical impedance spectroscopy (EIS) analysis of full capacitors with 2% and 5% LiDFOB additions showed that excessive LiDFOB (5%) increased the charge transfer resistance, explaining the lower energy efficiency observed at higher additive concentrations. The optimal concentration of 2% achieved the best balance between SEI stabilization and minimizing interfacial resistance.
The degradation of the AC//HC full capacitor without pre-lithiation was primarily attributed to the progressive consumption of Li⁺ from the electrolyte and the structural deterioration of the hard carbon anode due to unstable SEI formation. The optical photographs and SEM images clearly showed the detachment of hard carbon particles from the current collector in the non-pre-lithiated cell after cycling, whereas the pre-lithiated cell maintained integral electrode structure.
Conclusions and Perspectives
Based on my comprehensive investigation of cathode pre-metallization processes for high energy density energy storage battery systems, I draw the following conclusions:
For the lithium-ion capacitor pre-lithiation: The synergistic combination of Li₂NiO₂ as a sacrificial lithium reagent and LiDFOB as an electrolyte additive enables efficient pre-lithiation with remarkably low (20%) additive content. LiDFOB increases the utilization efficiency of Li⁺ released from LNO, addresses the Li₂NiO₂ limitation of low lithium supply capacity, and avoids excessive use of pre-lithiation reagents. The pre-lithiation process achieves a near-ideal potential distribution between the positive and negative electrodes, with the negative electrode potential maintained at 0.05-0.67 V after pre-lithiation. The LiF-rich SEI layer formed with LiDFOB enhances full capacitor energy efficiency to 80% and enables excellent high-rate cycling performance.
For the sodium-ion battery pre-sodiation: I have developed an efficient pre-sodiation reagent by uniformly loading NaHF₂ nanoparticles onto high-surface-area activated carbon through high-energy ball milling. The activated carbon provides a three-dimensional conductive network and catalytic active sites that promote the complete decomposition of NaHF₂ with a low decomposition potential of 3.7 V. The pre-sodiation reagent exhibits a high irreversible capacity of 310.4 mAh g⁻¹, and 5% addition effectively compensates for the first-cycle sodium loss in the LCMNMO//hard carbon full battery. The pre-sodiated sodium-ion battery achieves an ultra-high energy density of 274.4 Wh kg⁻¹ for high energy density energy storage battery applications, with excellent cycling stability.
My research offers new insights into the design and optimization of cathode pre-metallization processes for high energy density energy storage battery systems. Future work could explore the following directions: developing dual-functional additives that combine SEI formation capability with lithium/sodium supply through molecular design; investigating electrocatalytic approaches to further reduce the decomposition potential of sacrificial reagents; and extending the pre-metallization strategies to other emerging battery chemistries such as potassium-ion batteries and multivalent ion batteries.