High-capacity battery cathode prelithiation to
o set initial lithium loss
Yongming Sun1,Hyun-Wook Lee1,Zhi Wei Seh1,Nian Liu1,Jie Sun1,Yuzhang Li1and Yi Cui1,2*
Loss of lithium in the initial cycles appreciably reduces the energy density of lithium-ion batteries.Anode prelithiation is a common approach to address the problem,although it faces the issues of high chemical reactivity and instability in ambient and battery processing conditions.Here we report a facile cathode prelithiation method that o ers high prelithiation e cacy and good compatibility with existing lithium-ion battery technologies.We fabricate cathode additives consisting of nanoscale mixtures of transition metals and lithium oxide that are obtained by conversion reactions of metal oxide and lithium. These nanocomposites a ord a high theoretical prelithiation capacity(typically up to800mAh g−1,2,700mAh cm−3)during charging.We demonstrate that in a full-cell configuration,the LiFePO4electrode with a4.8%Co/Li2O additive shows11% higher overall capacity than that of the pristi
ne LiFePO4electrode.The use of the cathode additives provides an e ective route to compensate the large initial lithium loss of high-capacity anode materials and improves the electrochemical performance of existing lithium-ion batteries.
R echargeable lithium(Li)-ion batteries at present dominate the portable electronics market and exhibit great potential for electric vehicles,grid-scale energy storage and renewable energy storage1–5.Commercial Li-ion batteries are composed of two electrodes:an intercalated lithium transition metal oxide cathode (for example,LiCoO2,LiMn2O4and LiFePO4)and a graphite anode.During the first charge process,5–20%of the Li from the cathode is usually consumed owing to the solid electrolyte interphase(SEI)formation at the anode surface,leading to low first-cycle Columbic efficiency and a high initial irreversible capacity loss6–12.Electrochemical prelithiation routes,such as directly placing a Sn–C electrode in direct contact with a Li foil wetted by the electrolyte solution13and discharging a Si nanowire electrode/Li foil half cell14,showed effective Li compensation efficiency.In spite of their success,such complex operation processes and their instability in ambient atmosphere were not suitable for scale-up. Another method to compensate the first-cycle Li loss is to load additional cathode materials.However,owing to the low specific capacity(<200mAh g−1)of existing cathode materials,a large amount of additional loading is needed,which reduces appreciably the specific energy and energ
y density of the entire battery.This challenge calls for specifically designed prelithiation additives as high-specific-capacity Li donors to offset the initial Li loss.Some progress has been made in the design of such prelithiation materials on both anodes and cathodes.Stabilized Li metal powders and Li silicide nanoparticles were added to the anodes for prelithiation15–17. The irreversible capacity of the anodes can be compensated by a small amount of such additives.However,anode prelithiation materials have a low potential and high chemical reactivity,creating compatibility issues with ambient environments,common solvents, binders and thermal processing in Li-ion batteries.Cathode prelithiation is another route to compensate the Li loss in the battery (Supplementary Fig.1).Sacrificial Li salt additives(for example, azide,oxocarbons,dicarboxylates and hydrazides)exhibited Li compensation effects for the first irreversible capacity loss.However, as mentioned in ref.18,the use of these additives was accompanied by the evolution of undesired gaseous N2,CO or CO2.Similarly,
Na3N was used as an additive in sodium-ion batteries with
concomitant N2evolution19.Moreover,some earlier work on Li-
rich compounds as cathode additives(for example,Li2NiO2and
Li6CoO4)has been carried out20,21.In spite of the progress,the
effective compensation of the first-cycle Li loss is still limited by
their low specific capacity(∼300mAh g−1).In this work,we develop
a one-pot chemical reaction route to synthesize nanocomposites of
lithium oxide(Li2O)and metal(M)with deep nanoscale mixing for
battery cathode prelithiation.Owing to the unique characteristics
of the large potential hysteresis of the conversion reaction,the
lithium in such nanocomposites can be easily extracted on charging,
whereas it does not transform back to the initial state on discharging.
Therefore,the as-synthesized M/Li2O nanocomposites deliver a
high‘donor’Li-ion capacity.As an example,we show that the
overall capacity of a LiFePO4cathode with the Co/Li2O additive is
appreciably improved in a full-cell configuration.
Design for cathode prelithiation
To search for an effective cathode prelithiation additive,we believe
that the following points can serve as good guidelines.First,a good
cathode additive should possess a much higher Li storage capacity by
weight and volume than the existing cathode materials.For example,
if we choose doubling of the existing cathode materials as a criterion,
we would need to find a prelithiation additive with capacity >400mAh g−1or>1,200mAh cm−3.Second,the additive should be able to release its stored Li ions below the maximum potential
during cathode charge,but not take in Li ions at the minimum
potential of cathode discharge(Fig.1a).That is,we would need the
delithiation potential of additives to be below the maximum cathode
charge potential while the lithiation potential of additives should be
below the minimum cathode discharge potential.This implies that
the delithiation/lithiation potential curve of additives should have
a large hysteresis.Third,the cathode prelithiation additives should
not have intolerable negative effects on the stability of electrode
materials,electrolyte,and the whole battery.Usually,that means
1Department of Materials Science and Engineering,Stanford University,Stanford,California94305,USA.2Stanford Institute for Materials and Energy Sciences,SLAC National Accelerator Laboratory,2575Sand Hill Road,Menlo Park,California94025,USA.*e-mail:yicui@stanford.edu
DOI:
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+/Li 0
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L i F e P O 4LiFePO 4
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o /L i 2O  (1/2)R u /L i 2O  (1/2)M n /L i 2O  (1/1)N i /L i 2O  (1/1)C o /L i 2O  (1/1)C u /L i 2O  (1/1)P b /L i 2O  (1/2)L i F e P O 4C u /L i 2O  (2/1)M n /L i 2O  (1/2)
M o /L i 2O  (1/3)M n /L i 2O  (2/3)F e /L i 2O  (2/3)M n /L i 2O  (3/4)F e /L i 2O  (3/4)C o /L i 2O  (3/4)M o /L i 2O  (1/2)R u /L i 2O  (1/2)M n /L i 2O  (1/1)N i /L i 2O  (1/1)C o /L i 2O  (1/1)Co 3O 4C u /L i 2O  (1/1)P b /L i 2O  (1/2)a
b
c
d
Figure 1|Schematic of M/Li 2O composite cathode additives for a Li-ion battery.a ,Potential requirement for ideal cathode additives:complete
delithiation below the cuto charge potential of the cathodes and starting lithiation below their cuto discharge potential.b ,Theoretical specific capacities and volumetric capacities of various M/Li 2O composites based on the inverse conversion reaction.c ,Typical potential curves of MOs (for example,
Co 3O 4)and existing commercial cathodes (for example,LiFePO 4).The Li in M/Li 2O composites can be extracted below the cuto charge potential of the cathodes but not consumed by MOs above their cuto discharge potential.d ,Required amount of M/Li 2O cathode prelithiation materials to achieve complete Li compensation in comparison to that using an extra amount of cathode material (in the case of a LiFePO 4/graphite full cell with an initial irreversible Li loss of 30mAh g −1).
a relatively high open-circuit voltage (OCV)is needed (Fig.1a).Fourth,the cathode prelithiation additives should ideally be stable in ambient conditions and compatible with existing industrial battery fabrication processes such as slurry mixing,coating and baking.With the above criteria in mind,we identify the reaction products of transition metal oxides (MOs)with Li to be excellent candidates as cathode prelithiation additives.The MOs (M x O y ,M =Fe,Co,Ni,Mn,and so on)have been intensively investigated as potential anode materials over the past decade 22–27.They can react with Li through a conversion-reaction mechanism (M x O y +2y Li ++2y e −→x M +y Li 2O)to form nanocomposites of M and Li 2O.The M/Li 2O nanocomposites store more than four times the theoretical specific capacity of existing cathodes (for example,724mAh g −1for Co/Li 2O (molar ratio,3/4),799mAh g −1for Fe/Li 2O (molar ratio,2/3),and 935mAh g −1for Mn/Li 2O (molar ratio,1/2),see Fig.1
b and Supplementary Table 1).On the basis of the density of M and Li 2O,they can also deliver high vol
umetri
c capacities (for example,2,695mAh cm −3for Co/Li 2O (molar ratio,3/4),2,735mAh cm −3for Fe/Li 2O (molar ratio,2/3)an
d 2,891mAh cm −3for Mn/Li 2O (molar ratio,1/2),se
e Fig.1b and Supplementary Table 1).
The conversion reactions of MOs usually exhibit a lithiation potential below    1.2V and a near complete delithiation at a potential below ∼3V ,which show a large charge/discharge voltage hysteresis.For example,Fig.1c shows the experimental first-cycle electrochemical lithiation/delithiation voltage curve of Co 3O 4,where lithiation mainly takes place between 1.2to 0.01V and delithiation can provide a very high Li-ion capacity of 842mAh g −1below 3V .The wide voltage range and large hysteresis make conversion oxides alone not ideal either as good anode or cathode materials 28,29.However,these very characteristics of conversion oxides make them excellent prelithiation additives for cathodes.The charge cutoffpotential of existing cathode materials is usually larger than 4.0V versus Li/Li +(for example,4.2V for LiFePO 4and 4.3V for LiCoO 2,Fig.1c),which is high enough to easily
extract the Li from the M/Li 2O composites.Meanwhile,their discharge cutoffpotential is higher than 2.5V (for example,2.5V for LiFePO 4and 3.0V for LiCoO 2,Fig.1c),which is still much higher than the lithiation potential of MOs.In other words,M/Li 2O composites as cathode additives can contribute a large amount of Li during the first charge process of cathodes,whereas their lithiation reaction cannot occur in the cathode discharge process.The impact of having M/Li 2O composites as prelithiation additives is shown in Fig.1d,which shows the amount of additional material needed to compensate the first-cycle Li loss (for example,30mAh g −1)in a LiFePO 4/graphite full cell.One has to load an additional 18%of LiFePO 4cathode material.In contrast,only 3–5%of M/Li 2O cathode additives is needed to offset this initial Li loss (Fig.1d).Correspondingly,the specific and volumetric energy densities are increased to 8–13%and 11–14%,respectively (Supplementary Fig.2).
Electrochemistry of the Co/Li 2O nanocomposite
The above promising analysis on using M/Li 2O composites as cathode prelithiation additives has motivated us to experimentally demonstrate such a possibility and uncover the unknowns related to this novel concept.In this work,we develop a general one-step synthesis method for M/Li 2O (M:Co,Fe,Ni,et al.)nanocomposites (Fig.2a)and demonstrate their successful application as cathode additives to effectively compensate the Li loss during the first charge process in Li-ion batteries.A nano
metre-sized metal/nanometre-sized lithium oxide (N-M/N-Li 2O)composite was synthesized by mixing M x O y with molten Li under Ar atmosphere.The molar ratio of the starting M x O y and Li was set according to the conversion-reaction equation (M x O y +2y Li →x M +y Li 2O).Li metal foil was melted at 185◦C and reacted with M x O y for 20min under mechanical stirring.Then,the temperature was further increased to 200◦C and kept for 2h for their complete reaction.Finally,a uniform nanocomposite comprising nanometre-sized
DOI:10.1038/NENERGY.2015.8
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SM -Co/N-Li 2O N-Co/N-Li 2O Capacity (mAh g −1 Co/Li 2O)
Capacity (mAh g −1 Co/Li 2O)
Figure 2|Fabrication and electrochemical characteristics of the N-Co/N-Li 2O composite.a ,Schematic of the fabrication process of the N-M/N-Li 2O composites.MOs are used as the starting materials and in situ converted into N-M/N-Li 2O composites through the chemical reaction with molten Li.
b ,Initial charge potential profiles of the electrodes made with various Co/Li 2O nanocomposites:M-Co/N-Li 2O composite,SM-Co/N-Li 2O composite and N-Co/N-Li 2O composite.
c ,Charge/discharge potential profiles of the N-Co/N-Li 2O electrode after the first charge process.
d ,Initial charg
e potential profiles o
f LiFePO 4electrodes with di erent amounts of the N-Co/N-Li 2O additive in half-cell configurations.e ,f ,The initial charge/discharge potential profiles (e )and cyclin
g performance (f )of LiFePO 4/graphite full cells wit
h and without the N-Co/N-L
i 2O additive.The specific capacities of the cathodes are evaluated on the basis of the weight of LiFePO 4and the N-Co/N-Li 2O additive.
metal particles embedded in a nanometre-sized Li 2O matrix was obtained (Fig.2a).These M/Li 2O nanocomposites are used in regular slurry processing in ambient conditions to form battery electrodes (see Methods).
As a typical example,the electrochemical performances of the nanometre-sized Co/nanometre-sized Li 2O (N-Co/N-Li 2O;molar ratio,3/4)composite were investigated (Fig.2b,c).As expected,the as-prepared N-Co/N-Li 2O composite delivered a high charge specific capacity of 619mAh g −1and a low discharge specific capacity of 10mAh g −1for the first cycle in the potential range between 4.1and 2.5V ,meeting the voltage criterion as a cathode prelithiation additive.For comparison,the electrochemical properties of two other Co/Li 2O nanocomposites,including micrometre-sized Co/nanometre-sized Li 2O (M-Co/N-Li 2O)and sub-micrometre-sized Co/nanometre-sized Li 2O (SM-Co/N-Li 2O)composites were also investigated (Fig.2b).They exhibited a much higher charge potential (∼4.1V)and a lower specific c
harge capacity (<100mAh g −1),owing to the large particle size of Co and the loose contact between the Li 2O and Co particles (discussed later).These results highlight the importance of having thorough nanoscale mixing of Co and Li 2O.Note that slurry coating and electrode baking of the M-Co/N-Li 2O composite were carried out in ambient conditions.The Li-extraction specific capacity was 568mAh g −1after an eight-hour exposure to ambient atmosphere,only 51mAh g −1lower than the initial value.Even after two days,the Li-extraction specific capacity still reached 418mAh g −1,suggesting compatibility
with the conventional Li-ion battery fabrication environment (Supplementary Fig.3).
The Li-extraction potential of the N-Co/N-Li 2O composite made by the one-step chemical reaction (Fig.2b)is slightly higher than that produced by the electrochemical lithiation product of Co 3O 4(Fig.1c).This might result from the different grain sizes of Co/Li 2O nanocomposites produced by these two methods.In contrast to the counterpart produced by electrochemical lithiation (Fig.1c),the chemically synthesized N-Co/N-Li 2O composite exhibits a high OCV of 1.8V (Fig.2b),which is high enough to be in the stability range of existing cathode materials.Thus it can be easily mixed with various cathode materials with negligible negative effects on their chemical and structural stability during electrode processing.More-over,the N-Co/N-Li 2O electrode lost nearly all its capacity after the f
irst cycle,suggesting that after providing Li ions during the first charge,these nanocomposites did not contribute to the active elec-trochemical process in the cathode (Fig.2c).All these characteristics enable the N-Co/N-Li 2O cathode additive to effectively compensate for the first-cycle capacity loss in existing Li-ion batteries.
Figure 2d shows the charge potential profiles of LiFePO 4cathode and Li metal anode half cells with different amounts of the N-Co/N-Li 2O cathode prelithiation material.Here,the specific capacity was calculated on the basis of the total weight of LiFePO 4and the N-Co/N-Li 2O composite unless otherwise stated.The electrode made of pristine commercial LiFePO 4powder delivers an initial charge capacity of 164mAh g −1.With a 4.8%N-Co/N-Li 2O additive (based on the entire cathode),the first-cycle
DOI:
10.1038/NENERGY.2015.8
Distance (nm)
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Figure 3|Structure and evolution of the N-Co/N-Li 2O composite on the Li-extraction process.a ,STEM image of the N-Co/N-Li 2O composite,showing that Co nanoparticles are uniformly embedded in a Li 2O matrix.b ,c ,HRTEM image (b )and the corresponding fringes (c )of a Co particle,showing its
crystallinity and the small particle size.d ,e ,Schematic illustration (d )and configuration (e )of the in situ TEM device.The N-Co/N-Li 2O composite on a Cu tip serves as the working electrode and a template-fabricated hollow carbon fibre functions as the counter electrode.f ,g ,Time-lapse TEM images (f )and EELS spectra (g ).After the voltage bias application,Li is extracted from the N-Co/N-Li 2O working electrode and transfers to the carbon counter electrode,leading to volume shrinkage.An N-Co/N-Li 2O particle aggregate is labelled by a red circle and its shrinkage in volume is observed.EELS spectra are taken at the hollow carbon fibre,labelled by a pink point in e ,before and after the delithiation of the N-Co/N-Li 2O composite.The results indicate the transfer of Li from the working electrode to the counter electrode.
specific charge capacity reaches 183mAh g −1(12%higher).When an 8%N-Co/N-Li 2O additive is used,
the initial specific charge capacity is as high as 195mAh g −1(19%higher).A potential slope above the charge plateau of the pristine LiFePO 4is clearly observed,which is consistent with the delithiation process of the pristine N-Co/N-Li 2O composite (Fig.2b,d).The response of the additive at different potential ranges is clearly shown by a LiFePO 4electrode containing a large amount of the N-Co/N-Li 2O additive (with the additive/LiFePO 4ratio of 1/6in weight,Supplementary Fig.4).Meanwhile,the LiFePO 4cathodes with and without the N-Co/N-Li 2O additive in the Li metal half cells show high specific discharge capacities and stable cycling performance (Supplementary Fig.5),indicating that the N-Co/N-Li 2O additive has negligible negative effects on the stability of the cathode during cycling.Anodes with prelithiation materials can form better passivating surface films on their surfaces than that of the pristine anodes,which may help achieve stable electrochemical performance
for the anodes 16.A recent paper reported that a stable surface film on the LiNi 0.5Mn 0.3Co 0.2O 2cathode surface could also be formed by cathode prelithiation activation,which improved the cycling stability of the LiNi 0.5Mn 0.3Co 0.2O 2cathode 30.To show the generality of using the N-Co/N-Li 2O additive,we have also tried LiCoO 2and LiNi 0.6Co 0.2Mn 0.2O 2cathodes with such an additive to demonstrate its Li ‘donor’effect.The first charge potential profiles of LiCoO 2and LiNi 0.6Co 0.2Mn 0.2O 2with and without the N-Co/N-Li 2O additive are shown in Supplementary Fig.6.The Li ‘donor’effe
ct of the additive is clearly confirmed by the variations in potential slope caused by the additives and the increased overall capacities (based on the mass of cathode material and additive).The initial charge capacity for the LiCoO 2electrode with a 4.8%N-Co/N-Li 2O additive is 190.8mAh g −1(12%higher than 170.2mAh g −1for the pristine LiCoO 2electrode).With a 4.8%N-Co/N-Li 2O additive,the first-cycle specific charge capacity of the LiNi 0.6Co 0.2Mn 0.2O 2cathode reaches 208.0mAh g −1(9%higher than that of the pristine
DOI:10.1038/NENERGY.2015.8
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Figure 4|Generalization to other N-M/N-Li 2O composites.a ,b ,Initial charge/discharge potential profiles of the pristine N-Ni/N-Li 2O (a )and
N-Fe/N-Li 2O electrodes (b ).c ,d ,Discharge/charge potential profiles of LiFePO 4electrodes with 4.8%N-Ni/N-Li 2O (c )and N-Fe/N-Li 2O (d )additives in half-cell configurations.e ,f ,Cycling performance of LiFePO 4electrodes with 4.8%N-Ni/N-Li 2O (e )and N-Fe/N-Li 2O (f )additives.The specific capacities of the cathodes are evaluated on the basis of the weight of LiFePO 4and the additives.
LiNi 0.6Co 0.2Mn 0.2O 2electrode).The electrodes with and without the additive show similar cycling st
ability,indicating that the N-Co/N-Li 2O additive has negligible negative effects on the stability of various cathodes.
In a full cell,all the Li comes from the cathode materials.Owing to the irreversible reaction during the first-cycle charge process,a certain amount of Li cannot go back to the cathode during the discharge process,which reduces the specific energy and energy density of a battery.Ideally,when the amount of cathode additive is optimized,the discharge capacity of the cathode in a full cell can reach the same value as that obtained in a Li metal half cell.To fur-ther evaluate the Li compensation effect of the N-Co/N-Li 2O com-posite,electrochemical characterizations of LiFePO 4/graphite full cells were carried out.As shown in Fig.2e,the reversible discharge capacity of the as-made pristine LiFePO 4/graphite full cell without the N-Co/N-Li 2O cathode additive is only 120mAh g −1.In com-parison,a LiFePO 4/graphite full cell with a 4.8wt%N-Co/N-Li 2O cathode additive in the total cathode delivers a reversible discharge capacity of 133mAh g −1(11%higher),on the basis of the weight of LiFePO 4and the N-Co/N-Li 2O composite,which is the same value as that achieved in a LiFePO 4/Li metal half cell (Supple-mentary Fig.5).When calculated on the basis of the weight of
LiFePO 4,the reversible discharge capacity is as high as 141mAh g −1(Supplementary Fig.7).The first-cycle charge potential profiles of the LiFePO 4/graphite full cell with and without the N-Co/N-Li 2O addi
tive coincide well at a low charge potential (≤3.3V),and a prolonged charge plateau and slope are observed for the cell with the Co/Li 2O cathode additive at a high charge potential (≥3.3V).Cor-respondingly,the cell with the cathode additive shows a prolonged plateau during their discharge progress.These results confirm that the first-cycle capacity loss of a full cell is effectively compensated by the released capacity from the N-Co/N-Li 2O additive.The overall capacity and energy density of the entire Li-ion battery are appre-ciably improved.Furthermore,the full cell with a 4.8%cathode additive exhibits stable potential curves on cycling and significantly enhanced capacity retention with a small average capacity decay of 0.07%per cycle in 100cycles (Fig.2f and Supplementary Fig.8),which has comparable or better stability than the reference pristine LiFePO 4/graphite full cell without the N-Co/N-Li 2O additive.
Structure characterizations and in situ TEM measurement
Next,we sought to investigate the structure of the N-Co/N-Li 2O composite and its evolution during the electrochemical Li-extraction process.Results from X-ray photoelectron spectroscopy

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