Solid Electrolyte Interphase Research Articles

Tailoring the Electrode Interface Microenvironment to Stabilize Zn Metal Anode.

Zn metal is the most attractive anode material for aqueous batteries, yet it encounters challenges from dendrites. Here, based on lanthanum trifluoromethanesulfonate (La(OTf)3)-based electrolyte, the idea of tailoring the electrode interface microenvironment (ion concentration, solid electrolyte interphase (SEI) and electric field) is proposed to stabilize the Zn metal anode. The theoretical and experimental results show that the reconstruction of the electrolyte microstructure by OTf- and the capture of SO4 2- by La3+ enhance the liquid-phase mass transfer, which alleviates the ion concentration gradient on the anode surface. Meanwhile, the electrolyte decomposes to form a favorable inorganic-rich SEI. Importantly, the adsorbed La3+ homogenizes the electric field intensity at the tip of the anode surface. Benefiting from the improved interface microenvironment, the Zn electrodeposition behavior is efficiently regulated, endowing the self-elimination behavior of the regenerated dendrites. As a proof-of-concept, the Zn metal anode shows a highly reversible plating/stripping cycling in both Zn||Cu (7000 cycles) and Zn||Zn cells (3600h). Also, the NH4V4O10||Zn pouch cell operates stably for over 500 cycles and exhibits a low-gassing behavior.

Prelithiating Si-Based Anodes using Li-Excess Layered Positive Electrode Materials

Abstract With the increasing commercialization of silicon-based anodes, their high first-cycle irreversible capacity becomes a critical issue to address. Future cycles may also require additional lithium due to the evolution of the anode's solid electrolyte interphase. This work introduces Li-excess layered Li1.11(Ni0.5Mn0.5)0.89O2 (Li-excess NMC550) as suitable cathode choice to provide additional lithium reserves to the cell. The excess lithium can be removed from the cathode structure irreversibly when the cell is subjected to voltages greater than 4.4V, making it a good choice for prelithiating Si-based cells without requiring any additional processing. In addition, the Li-excess NMC550 does not display voltage fade typical of Li-rich and Mn-rich materials with more Mn than Ni atoms in the structure. This strategy is shown in this work well in NMC550: silicon/carbon (Si/C) cells. Cells were cycled to an initial upper cutoff of 4.6 V to transfer excess lithium from the cathode to the Si-based anode, followed by typical cycling within a stable voltage window. An 11% excess lithium reserve enhances cell energy density and prevents early capacity loss associated with lithium inventory depletion.

Controllable SiOx Coating Layer Promotes High Stable Si Anode for Lithium-Ion Batteries.

Silicon (Si) is considered as one of the most promising candidates for next-generation lithium-ion batteries with high energy density. The main problems are the severe volume expansion and continuous interfacial side reaction of Si that hinder its further application. It can be an effective way by constructing a robust coating layer outside of Si to impede/alleviate the above effect. SiOx with high mechanical strength can largely promote the electrochemical performance of Si. Herein, Si@SiOx material with high specific surface area, high porosity, and controllable coating was synthesized via a simple solid-liquid reaction by LiOH solution etching effect. The etching/oxidation mechanism of Si under alkaline conditions was thoroughly investigated. The surface oxide layer of Si was beneficial to the formation of a solid electrolyte interphase (SEI) with excellent stability and high Li+ conductivity, while its high-porosity structure reduces the volume expansion of the material by approximately 110%. Under the synergistic effect of etching-oxidation, the modified material exhibited superior electrochemical properties. When employed as anode materials, the specific capacity was as high as 3101.5 mAh g-1 and maintained at 841.0 mAh g-1 after 500 cycles at 1 A g-1.

Electrochemical formation of bis(fluorosulfonyl)imide-derived solid-electrolyte interphase at Li-metal potential.

Lithium bis(fluorosulfonyl)imide-based liquid electrolytes are promising for realizing high coulombic efficiency and long cycle life in next-generation Li-metal batteries. However, the role of anions in the formation of the solid-electrolyte interphase remains unclear. Here we combine electrochemical analyses and X-ray photoelectron spectroscopy measurements, both with and without sample washing, together with computational simulations, to propose the reaction pathways of electrolyte decomposition and correlate the interphase component solubility with the efficacy of passivation. We discover that not all the products derived from interphase-forming reactions are incorporated into the resulting passivation layer, with a notable portion present in the liquid electrolyte. We also find that the high-performance electrolytes can afford a sufficiently passivating interphase with minimized electrolyte decomposition, by incorporating more anion-decomposition products. Overall, this work presents a systematic approach of coupling electrochemical and surface analyses to paint a comprehensive picture of solid-electrolyte interphase formation, while identifying the key attributes of high-performance electrolytes to guide future designs.

Probing a solid electrolyte interphase layer with sub-nanometer pores using redox mediators

The solid electrolyte interphase (SEI) layer is crucial for lithium-ion batteries and has a significant impact on the electrochemical performance of negative electrodes, particularly for conversion-type materials with large volume changes and metallic lithium anode. However, the SEI layer has not yet been well understood. In this work, we used redox mediators of various sizes to probe the SEI layer that formed in carbonate-based electrolytes. The SEI layer has diffusion channels that allow the mediators smaller than benzoquinone (5.7 Å) to pass, suggesting that lithium ions have to partially de-solvate to pass through. Additionally, due to partial desolvation, the diffusion coefficient in the diffusion channels was higher than that in the bulk electrolytes. Both lithium salts and solvents influenced the size and areal density of channels. Herein, we aim to enhance comprehension of SEI structure and provide a method to study porous SEI layers using mediators, which can be extended to other electrochemical systems.

Design of a LiF-rich solid electrolyte interphase layer through a fluorinated carbon (CFX) complex separator for stable lithium metal batteries

The formation of a stable solid electrolyte interphase (SEI) layer is very important for improving the cycling stability and safety of lithium metal batteries (LMBs). However, since the reactivity of lithium metal anodes (LMAs) is very high, controlling the movement of Li+ at the anode/electrolyte interface remains challenging. In this study, an approach involving coating a fluorine functional-controlled fluorinated carbon (CFX) layer onto a commercial PE separator to form a stable SEI layer was proposed. The strong reaction between the fluorine functional groups constituting CFX and Li+ facilitated the rapid formation of a LiF-rich SEI layer in the resting and initial cycling stages. This initial stable SEI layer promoted a subsequent homogeneous Li+ flux, thus improving the LMA stability. In addition, the mechanism by which the total amount of fluorine and the fluorine functional groups control the Li+ dynamics through the CFX-coated PE separator with controlled fluorine functional groups was used to identify the mechanism by which the total amount of fluorine and the fluorine functional groups provide the advantage of the creation of a stable SEI layer. Therefore, this study contributes to the energy storage field by solving the cycling stability problem related to LMAs and emphasizes that a stable SEI layer can be formed based on the important interface control according to the type of fluorine functional group.

Constructing valid Li+ fast-transfer channels on novel P(TPC@Lys-Li) separator to enable security, high energy density, and controllable Li dendrites for lithium-metal batteries

Lithium metal batteries (LMBs) attract widespread attention under current high energy density developments. However, wild Li dendrite propagations owing to chaotic Li depositions raise challenges for LMB blossoming. Separators with specific demands thus should be involved when smoothly transferring into LMBs. In this research, a fascinating P(TPC@Lys-Li) separator is prepared by the turbulent interfacial polymerization and electrospinning to ingeniously clarify dependencies of Li+ transfer dynamics on double electric layer thickness (DELT) regulations within porous skeletons. P(TPC@Lys-Li) enables negatively charged porous skeleton surface and compresses DELT, which constructs high-efficiency Li+ fast-transfer channels on porous skeletons and accelerates Li+ transfer speed by 18.3 times. Especially, P(TPC@Lys-Li) supplies extra Li+ for stable formations of solid electrolyte interphase (SEI) layer, homogenizing nucleation and growth behaviors during Li depositions. Remarkable C-rate capacity and cycle stability thus arise for assembled LMBs, holding 87.2 % capacity retention after 700 cycles at 0.5C even under high cathode loading and lean electrolyte conditions. P(TPC@Lys-Li) also maintains constant physical scale and unabated mechanical strength even at elevated temperatures approaching 200 °C, which provides excellent battery security as LMBs encounter uncontrollable thermal runaway. Above attractive features enable P(TPC@Lys-Li) separator to be potentially applied in LMBs demanding sufficient security, high-capacity density, and fast charge technology.

2,2-Dimethylvinylboronic acid as an effective solid electrolyte interface formation additive for high performance sodium ion batteries

The main challenges brought by the unstable solid electrolyte interface (SEI) film seriously hinder the practical application of sodium ion batteries (SIBs). Herein, 2,2-dimethylvinylboronic acid (DEBA) is used as a film-forming electrolyte additive to improve the stability of the interface between the hard carbon (HC) and electrolyte for SIBs, naming the resulting electrode HCD-DEBA. DEBA is preferentially decomposed before the electrolyte and involved in forming SEI. Besides, in the cycling process, the C=C bonds in DEBA undergo the in-situ polymerization to construct the polymer skeleton of the SEI, inducing forming a thinner solid electrolyte interface layer with lower interface impedance, thus enhancing cycling stability. The optimized HCD-2%DEBA exhibits a first-cycle discharge specific capacity of 332.5 mAh g−1 at 0.1 C, while the HCD is 329.6 mAh g−1. HCD-2% DEBA was able to cycle 5000 times at a high rate of 5 C with capacities ranging from 195.7 mAh g−1 to 110.4 mAh g−1, however, the one without DEBA died after about 2000 cycles with an initial discharge capacity of 83.6 mAh g−1. This work provides a simple strategy for improving the cycle capacity of hard carbon half-cells and constructing the solid electrolyte interphase.

Room temperature reversible Ca-metal chemistry in commercial fluorinated calcium salt ester electrolytes enabled by a compact N-rich interphase layer

Multivalent metal batteries serve as crucial complements to lithium-ion batteries. Nevertheless, fluorinated anions designed for energy storage at high voltages readily passivate the metal anodes in common ester electrolytes, particularly for Ca-metal batteries (CMBs). Ca plating/stripping can hardly occur continuously in common fluorinated calcium salt ester electrolytes due to severe corrosion at room temperature. To address the challenging issue, a uniform artificial interface of calcium fluoride nitride nanocrystals (Ca2NF) and amorphous organic species is engineered in the top of Ca-metal electrodes. The compact inorganic/organic hybrid solid electrolyte interphase (SEI) layer avoided the direct reaction of Ca-metal and corrosive fluorinated anions, effectively preventing the continuous passivation of Ca-metal and expediting the diffusion kinetics of calcium ions at the interface. Therefore, for the first time, stable Ca plating/stripping at 0.005 mA cm−2 over 420 h in a commercial fluorinated calcium salt ester electrolyte is achieved for the Ca-metal electrode with the protective SEI, far superior to that of mere hours for the pristine Ca-metal. The enhanced reversibility is also substantiated in Ca-metal full batteries with biomass-derived carbon cathode, delivering stable cycling performance. This work underscores the importance of interface and interphase engineering as an effective avenue for advancing the development of CMBs.

Impact of Na-Carboxymethyl Cellulose Binder Type on Hard Carbon Performance and SEI Formation in Sodium-Ion Batteries.

The development of hard carbon (HC) electrodes using biobased binders, formulated in water solvent, is of great interest for Na-ion batteries. Five Na-carboxymethyl cellulose (CMC) binders with different molecular weights and degrees of substitution were investigated. The increase in the CMC molecular weight led to an increase in the volume of water necessary for slurry preparation and a decrease in the electrode mass loading. Moreover, the adhesion of the slurry was strongly impacted by the aluminum current collector properties. A high initial Coulombic efficiency, iCE (up to ∼90%), and reversible capacity (up to 334 mAh g-1 at C/10) were obtained in Na half-cells. Post-mortem XPS analyses performed on several electrodes under various conditions allowed a better understanding of the formation and composition of the solid electrolyte interphase (SEI) layer. The genesis of SEI was initially chemically driven: to a small extent, by HC and to a greater extent, by the Na metal. However, SEI formation was mainly governed by the electrochemical-driven degradation of the electrolyte salt and solvent. The SEI layer composed of an inorganic-rich core and an organic-rich shell significantly decreased the resistance of the HC electrode, allowing superior iCE while maintaining high capacity retention (96.2%), after 100 cycles at 1C.

Synergistic Effect Enables Aqueous Zinc‐Ion Batteries to Operate at High Temperatures

AbstractThe performance of aqueous zinc‐ion batteries (AZIBs) at high temperatures (HT) is severely compromised by active water corrosion, parasitic reactions, and dendrite growth. Herein, zinc trifluoroacetate is introduced at a low concentration (0.2 m), dissolved in triethyl phosphate (TEP)and H2O. The active water is suppressed due to the reconstructed original hydrogen bond network, which helps inhibit parasitic reactions and severe corrosion. Meanwhile, a solid electrolyte interphase (SEI) formed on the zinc anode due to the decomposition of the introduced zinc salt. The high‐tolerance SEI physically separates the electrolyte and anode, reducing the corrosion caused by active water. Moreover, TEP, as a prevalent fire‐retardant cosolvent, can preferentially anchor on the zinc sheet, serving as a shielding buffer layer. TEP is not only reconstructing the structure of the electric double layer (EDL), decreasing the content of active water, but also accelerating the prompt formation of SEI further. As proof of this synergistic effect, the assembled symmetric Zn.

Radiation Chemistry Reveals the Reaction Mechanisms Involved in the Reduction of Vinylene Carbonate in the Solid Electrolyte Interphase of Lithium-Ion Batteries.

A safe and efficient lithium-ion battery requires including an additive in the electrolyte. Among the additives used, vinylene carbonate (VC) is particularly interesting, because it leads to the formation of a stable and protective solid electrolyte interphase (SEI) on the negative electrode. However, the reduction behavior of VC, resulting in polymer formation, is complex, and many questions remain as to the corresponding reaction mechanisms. In particular, in conventional battery studies, it is not possible to observe the transient species formed during reduction. Using picosecond pulsed radiolysis coupled with theoretical chemistry calculations, we showed that, once formed, the anion radical VC·- can undergo ring opening in a few nanoseconds or form (VC)2·-.Within 100 ns, each of these anions then leads to the formation of (VC)(C3H2O3·-). This latter species starts oligomerizing. Eventually, a polymer is formed. Although it mainly consists of poly(VC) units, other chemical functions, such as alkyl groups, are also present, which highlights the role played by water, even in trace amounts. Lastly, we propose a scheme of the reaction mechanisms involved in VC reduction, leading to its polymerization. Clearly, the polymer formed from VC at the SEI of lithium-ion batteries has a complex structure.

Versatile Biopolymers for Advanced Lithium and Zinc Metal Batteries.

Lithium (Li) and zinc (Zn) metals are emerging as promising anode materials for next-generation rechargeable metal batteries due to their excellent electronic conductivity and high theoretical capacities. However, issues such as uneven metal ion deposition and uncontrolled dendrite growth result in poor electrochemical stability, limited cycle life, and rapid capacity decay. Biopolymers, recognized for their abundance, cost-effectiveness, biodegradability, tunable structures, and adjustable properties, offer a compelling solution to these challenges. This review systematically and comprehensively examines biopolymers and their protective mechanisms for Li and Zn metal anodes. It begins with an overview of biopolymers, detailing key types, their structures, and properties. The review then explores recent advancements in the application of biopolymers as artificial solid electrolyte interphases, electrolyte additives, separators, and solid-state electrolytes, emphasizing how their structural properties enhance protection mechanisms and improve electrochemical performance. Finally, perspectives on current challenges and future research directions in this evolving field are provided.

Slow Voltage Relaxation of Silicon Nanoparticles with a Chemo-Mechanical Core-Shell Model.

Silicon presents itself as a high-capacity anode material for lithium-ion batteries with a promising future. The high ability for lithiation comes along with massive volume changes and a problematic voltage hysteresis, causing reduced efficiency, detrimental heat generation, and a complicated state-of-charge estimation. During slow cycling, amorphous silicon nanoparticles show a larger voltage hysteresis than after relaxation periods. Interestingly, the voltage relaxes for at least several days, which has not been physically explained so far. We apply a chemo-mechanical continuum model in a core-shell geometry interpreted as a silicon particle covered by the solid-electrolyte interphase to account for the hysteresis phenomena. The silicon core (de)lithiates during every cycle while the covering shell is chemically inactive. The visco-elastoplastic behavior of the shell explains the voltage hysteresis during cycling and after relaxation. We identify a logarithmic voltage relaxation, which fits with the established Garofalo law for viscosity. Our chemo-mechanical model describes the observed voltage hysteresis phenomena and outperforms the empirical Plett model. In addition to our full model, we present a reduced model to allow for easy voltage profile estimations. The presented results support the mechanical explanation of the silicon voltage hysteresis with a core-shell model and encourage further efforts into the investigation of the silicon anode mechanics.

Unraveling the Multifunctional Mechanism of Fluoroethylene Carbonate in Enhancing High‐Performance Room‐Temperature Sodium‐Sulfur Batteries

Room‐temperature sodium‐sulfur (RT Na‐S) batteries has attracted growing attentions in large‐scale energy storage technology, while the serious shuttle effect and interface side reaction limit its practical application. Despite fluoroethylene carbonate (FEC) has been widely used as an electrolyte additive or co‐solvent to facilitate the optimization of electrode‐electrolyte interphase in RT Na‐S batteries, its crucial influence and mechanism have not been clearly understood. Herein, we deeply reveal the two‐steps cathode‐electrolyte interphase (CEI) formation by using FEC as the exclusive electrolyte solvent. The results demonstrate that FEC participates in both rapid nucleophilic reaction and electrochemical decomposition on cathode, which can effectively in situ construct a unique "chocolate‐cookie" shaped CEI consisted of NaF‐rich double layers. This CEI eliminates the shuttle effect and ensures the solid‐solid conversion of sulfur. Along with the stable F‐rich solid‐electrolyte interphase (SEI) formed on the Na anode, the RT Na‐S battery delivers an impressive performance (456 mAh g‐1 after 1000 cycles at 0.5 C) with almost 100% Coulombic efficiency. This significantly simplifies electrolyte design and provides valuable insights into the application of FEC in practical Na‐S batteries

Unraveling the Multifunctional Mechanism of Fluoroethylene Carbonate in Enhancing High-Performance Room-Temperature Sodium-Sulfur Batteries.

Room-temperature sodium-sulfur (RT Na-S) batteries has attracted growing attentions in large-scale energy storage technology, while the serious shuttle effect and interface side reaction limit its practical application. Despite fluoroethylene carbonate (FEC) has been widely used as an electrolyte additive or co-solvent to facilitate the optimization of electrode-electrolyte interphase in RT Na-S batteries, its crucial influence and mechanism have not been clearly understood. Herein, we deeply reveal the two-steps cathode-electrolyte interphase (CEI) formation by using FEC as the exclusive electrolyte solvent. The results demonstrate that FEC participates in both rapid nucleophilic reaction and electrochemical decomposition on cathode, which can effectively in situ construct a unique "chocolate-cookie" shaped CEI consisted of NaF-rich double layers. This CEI eliminates the shuttle effect and ensures the solid-solid conversion of sulfur. Along with the stable F-rich solid-electrolyte interphase (SEI) formed on the Na anode, the RT Na-S battery delivers an impressive performance (456 mAh g-1 after 1000 cycles at 0.5 C) with almost 100% Coulombic efficiency. This significantly simplifies electrolyte design and provides valuable insights into the application of FEC in practical Na-S batteries.

Sodiophilic Interface and Electrolyte Regulation Boost the Lifespan of Anode‐Free Sodium Battery

ABSTRACTAnode‐free sodium batteries (AFSBs) have attracted increasing attention for their high energy density. However, they suffer from rapid capacity decline resulting from sodium dendrite growth at the sodium/host interface and irreversible side reactions at the electrolyte/sodium interface. Herein, a GaInSn‐coated Cu foil (G‐Cu), prepared by a simple brush coating method, was applied as the sodiophilic current collector to regulate sodium nucleation behavior. In addition, a nonexpendable functional electrolyte additive, hexamethyldisiloxane (HMDSO), was introduced, which could be absorbed on the sodium surface and serve as a protective layer against corrosion side reactions at the electrolyte/sodium interface. It is interesting to note that this additive barely participated in forming the solid electrolyte interphase. The synergetic effects of sodiophilic interface design and electrolyte regulation enable reversible sodium plating and stripping. Ultimately, the AFSB assembled using G‐Cu and HMDSO electrolyte with a highly loaded Na3V2(PO4)3 cathode (≈12.5 mg cm−2) delivers a discharge capacity of 84.5 mAh g−1 after 200 cycles with a high capacity retention of 87.6%, significantly extending its operation lifespan.

Enhanced salt content and stabilized bilayer passivation interphase toward long-life and high-safety anode-free lithium battery

Anode-free lithium batteries (AFBs) offer optimal energy density, low cost, and simple manufacturing, making them desirable for next-generation high-energy-density batteries. However, the continuous consumption of electrolyte and Li metal through the undesirably constructed solid electrolyte interphase (SEI) significantly limits the cycling and thermal stability of AFBs. To address this challenge, dual-solvent localized high-LiFSI salt concentration electrolytes (DS-LHCEs) are designed by the addition of sulfolane (SF) due to its higher LiFSI solubility and highly consistent Li+-solvent binding energy with DME. High LiFSI content increases the salt-to-solvent concentration, decreases the amount of free solvent molecules, and restricts the side reaction between the free solvent and Li metal. As a result, a flat and dense Li metal with a robust LiF-rich SEI is deposited on Cu foil. Moreover, the introduction of trioxane (TO) constructs a high-strength outer organic SEI layer, which protects the weak grain boundaries of the inner inorganic SEI layer, further enhancing the stability of the Li metal anode. This results in assembled AFB pouch cells with a superior lifetime (206 cycles with 80 % capacity retention) and a much higher onset self-heating temperature (119 °C vs. 58 °C in the DME-based baseline electrolyte).

Highly stable zinc anodes enabled by a dual-function additive of in-situ film formation and electrostatic shielding

The practical application of aqueous Zn-ion batteries (AZIBs) is significantly hindered by the short lifespan and low utilization of zinc metal anodes caused by the notorious hydrogen evolution reaction (HER), corrosion, and dendritic growth. The in-situ formation of a dense Zn-ion conducting solid electrolyte interphase (SEI) is an ideal approach to handle these problems. Herein, we present a twofold-protective strategy involving the dense in-situ SEI layer and the electrostatic shielding layer by incorporating 0.01 M yttrium nitrate (YN) additive into 2 M zinc sulfate electrolyte. The robust oxidizing characteristic of the nitrate induces rapid passivation on the surface of Zn anodes, by which these inevitable passivation byproducts are utilized for in-situ fabricating a dense SEI protection layer with excellent wettability. Besides, the infusion of high-valence rare-earth Y3+ forms a durable, dynamic electrostatic shielding layer around the growth tip of the protuberances. This can regulate Zn2+ deposition and aid in rectifying the irregular and sharp surface on zinc anodes. Benefiting from the synergistic effect of the static film protection and the dynamic electrostatic rectification, zinc anodes exhibit outstanding stability, as evidenced by remarkable cycling lifespan of exceeding 2000 h at 1 mAh cm−2 and 600 h at 10 mAh cm−2 in Zn symmetric cells and the preeminent CE of 99.83 % with stably cycling over 2200 times in half-cells at 4 mA cm−2. This study reveals a straightforward yet pragmatic avenue of synergistic dual-protection effect to achieve anti-corrosion and dendrite-free Zn anodes.

Improving Fast-Charging Performance of Lithium-Ion Batteries through Electrode-Electrolyte Interfacial Engineering.

The solid-electrolyte interphase (SEI) is a key element in anode-electrolyte interactions and ultimately contributes to improving the lifespan and fast-charging capability of lithium-ion batteries. The conventional additive vinyl carbonate (VC) generates spatially dense and rigid poly VC species that may not ensure fast Li+ transport across the SEI on the anode. Here, a synthetic additive called isosorbide 2,5-dimethanesulfonate (ISDMS) with a polar oxygen-rich motif is reported that can competitively coordinate with Li+ ions and allow the entrance of PF6 - anions into the core solvation structure. The existence of ISDMS and PF6 - in the core solvation structure along with Li+ ions enables the movement of anions toward the anode during the first charge, leading to a significant contribution of ISDMS and LiPF6 to SEI formation. ISDMS leads to the creation of ionically conductive and electrochemically stable SEI that can elevate the fast-charging performance and increase the lifespan of LiNi0.8Co0.1Mn0.1O2 (NCM811)/graphite full cells. Additionally, a sulfur-rich cathode-electrolyte interface with a high stability under elevated-temperature and high-voltage conditions is constructed through the sacrificial oxidation of ISDMS, thus concomitantly improving the stability of the electrolyte and the NCM811 cathode in a full cell with a charge voltage cut-off of 4.4V.