ndOBion

n,* arsity o

ion afB!. HF.g., PFd calcrolyter the fi

anionzation092# A

ived

trolyte and give rise to polarization. Furthermore, the anions must be 1:1 ion pairs was evaluated by conventional full ab initio Hartree-

e Let0 Telectrochemically stable. Many new lithium salts have been devel-oped that use the electron withdrawing power of fluorine atoms toachieve an anion with highly delocalized charge, the most extremeone comprising no less than 24 F atoms.4 There are, however, draw-backs with fluorine chemistry, as it is expensive and often harmful tothe environment.2 Therefore, attention has been drawn to alternativeanions, that do not resort to fluorine synthesis, for example, anionsbased on only carbon and nitrogen atoms.5,6

Recently, the group of Professor Angell has presented a numberof promising anions; bis~oxalate!borate ~BOB!, malonato-oxalato-borate ~MOB!, and bis~malonato!borate ~BMB!, based mainly onboron, carbon, and oxygen atoms.7,8 Here, we will examine the rela-tive lithium ion to anion interaction strengths of these anions vs.more traditional anions by ab initio quantum mechanical ~QM! cal-culations on 1:1 ion pairs. Traditional calculations in vacuum areused to shed light on interactions in solid polymer electrolytes ~e.g.,PEO-based!. To more closely mimic lithium salts dissolved in atypical high dielectric liquid electrolyte such as ethyl carbonate/polycarbonate ~EC/PC!, or indeed in gel-type electrolytes, a self-

Fock calculations with MP2 electron correlation added.13,14b

Results and DiscussionFirst we report data computed for ion pairs in vacuum, starting

with the GO approach, and, subsequently, we compare the resultswith those obtained by applying the SCRF method.9-11

In the GO method, the anion is treated as an effective fragmentpotential ~EFP! that contains terms representing exchange-repulsion,charge penetration, and energy gradients.14 The EFP is based on asingle ab initio calculation for the anion. The use of an EFP thatinteracts with the quantum mechanically treated lithium ion drasti-cally reduces the computational cost. Constructing the EFP fragmentuses only a few percent of the CPU-time of one single geometryoptimization step of an ab initio optimization. For the search forcoordination sites of the Li+ . 25 000 MC-steps can be made usingthe same CPU-time as a single ab initio geometry optimization step

aGas phase geometries were further optimized by the conductor polarizable continuummethod ~CPCM!9,10 implemented in the Gaussian 03 program with solvent = DMSO,

11Electrochemical Stability aof Orthoborate Anions (BPresentation of a Novel AnHenrik Markusson,z Patrik JohanssoDepartment of Applied Physics, Chalmers Unive

Ab initio calculations reveal the extraordinary low lithiummalonato-oxalatoborate ~MOB!, and bis~malonato!borate ~BMassociate with Li+ ~by ,10%! than most anions used today, eelectrolytes. Furthermore, by using self-consistent reaction fielthe applicability of these anions also for liquid and gel electoutlined and exemplified by a new anion, suggested here foelectrochemical stabilities in electrolytic solutions; all studiednew methodology to obtain starting data for geometry optimi 2005 The Electrochemical Society. @DOI: 10.1149/1.1869

Manuscript submitted March 9, 2004; revised manuscript rece

Today, Li-ion batteries are the systems of choice for a mobilesociety, offering high energy density, flexible and lightweight de-sign, and being environmentally friendly with a long lifespan. Thegel-type polymer electrolyte has been commercially used in batteriessince 19991 and is now the dominating battery concept in electricaldevices as cellular phones, digital cameras, etc. Important for furtherdevelopment of lithium based batteries are electrolytes with betterion conducting properties that are safe and provide sufficient me-chanical and electrochemical stability. Recently, hazardous reactionswere reported with the widely used PF6

anion ~e.g., as LiPF6 inlithium-ion batteries!,2 which is unstable ~facile release of PF5, astrong Lewis acid! and induces a cascade of unwanted side reactionsand safety problems.2 Together with the earlier reports of environ-mental persistence of TFSI-like compounds,3 there is an urge fordevelopment of new lithium salts for battery electrolytes.

As the ion conductivity in electrolytes is highly dependent on theconcentration of charge carriers and their mobility, a weak lithiumion coordinating anion is a necessity for obtaining a good electrolytewith high total ion conductivity. Anions that are less prone to asso-ciate with the lithium ion should be relatively large, with the chargedelocalized over several electronegative groups. The size is of im-portance as small anions more easily can diffuse through the elec-

Electrochemical and Solid-Stat1099-0062/2005/8~4!/A215/4/$7.0* Electrochemical Society Active Member.z E-mail: henrik.markusson@fy.chalmers.se

address. Redistribution su150.65.7.77Downloaded on 2013-04-18 to IP Lithium Ion-Anion Interactions, MOB, BMB), and: Tris-oxalato-phosphatend Per Jacobsson*

f Technology, SE-412 96 Gteborg, Sweden

finities of the orthoborate anions bis~oxalate!borate ~BOB!,and MP2 calculations reveal these anions to be less prone to

6

, BF4

, and TFSI, in low dielectric media such as polymericulations to model the effect of high dielectric solvents present,s was proven. Future directions of lithium salt synthesis arerst time. The HOMO levels of the anions are correlated tos can be anticipated to have wide stability ranges. Finally, a

s of cation-anion pairs by global optimization is presented.ll rights reserved.

December 20, 2004. Available electronically March 1, 2005.

consistent reaction field ~SCRF! technique was used.9-11a The gen-erality of this combined approach is used to suggest new improvedlithium salts for both polymeric and liquid electrolyte usage a priori,with the aim to optimize the synthesis efforts considerably, hereexemplified by a modification of the new orthoborate lithium salts.We will also introduce the use of effective fragment potentials~EFPs! to perform global optimization ~GO! of 1:1 ion pairs to lo-cate geometry optimization starting points.

The lithium ion to anion association can be analyzed by compar-ing the binding energies of different lithium ion coordination possi-bilities for each anion of choice. The strongest obtained interactiondetermines the association strength. A severe problem with ab initioQM calculations of lithium ion coordination to anions with a morecomplex geometry is the many degrees of freedom when choosingthe starting guesses. These starting guesses are crucial for generationof the final stable structures, but are difficult to make unambigu-ously and complete. In this study, we have searched the coordinationspace by use of Monte-Carlo simulations coupled with a simulatedannealing algorithm that slowly reduces the energy of the system,causing relaxation into minimum energy structures. Hereby, we ef-fectively eliminate the bias introduced by the starting guesses. Themethod was initially tested on the Li+-BF4

system, and subsequentlyapplied to the lithium ion pairs of the new orthoborate-based anions.The ability of the GO to locate the relevant starting guesses for the

ters, 8 ~4! A215-A218 ~2005!he Electrochemical Society, Inc.

A215 = 46.7, and using the Klamt radii model.bHartree-Fock energy optimization calculations were performed using the 6-31 G~d! and6-31 + Gsdd basis sets and electron correlation was added as MP2~full!/6-31 G~d!.

ecsdl.org/site/terms_usebject to ECS license or copyright; see

e Letuses. The total reduction in wall- and CPU-time is very large andtogether with the unambiguousness of the method emphasizes theusefulness of the strategy.

In general, we found the repulsion term of the EFP to be tooweak and therefore restricted the minimum cation-anion distance to1.7 . Using this strategy for the Li+-BF4 system starting guessescorresponding to both the expected energy minima were found.15c Inaddition, not only the unique minima were found. For this systemtotally 100 000 MC-steps were used in a single GO/EFP run.

For the orthoborate salts, several starting points were located bythe GO method. For LiBOB the global energy minimum and twoadditional local minima were found ~Fig. 1a, positions A-C!. Thedifferences in binding energy between these ion pairs are significant~Table I!. In total, five different local minima for Li+ coordination tothe MOB anion were found ~Fig. 1b, positions A-E!. MOB has aflexible six-membered ring and should thus permit geometry rear-rangements of the boron-oxygen tetrahedron, which may affect theusefulness of the GO/EFP method to locate all the relevant startingguesses. Indeed, upon Li+ coordination the geometry of the MOBanion changes significantly compared to that of the free anion.The EFP fragment of the GO is structurally inert and thus no sucheffects can be accounted for. Therefore, the minimum E was foundby hand using traditional methods. The BMB anion having twosix-membered rings is even more flexible than MOB. In addition tothe minima found by the GO approach ~C and D! we also found thebi-dentate lithium ion coordination to two carbonyl oxygen atomsfrom different rings ~Fig. 1c, A2!. The BMB anion becomes heavilydistorted upon complex formation.

Bidentate lithium ion coordination to two carbonyl oxygen atomsis found to be the most stable ion pairs for all complexes. The BOBanion is the most weakly coordinating orthoborate anion with abinding energy ,15 and ,30 kJ mol1 lower than for MOB andBMB, respectively ~Table I!. The partial loss of conjugation inMOB, due to the CH2-group, renders Li+-MOB a more stable ionpair despite the same coordination situation as in Li+-BOB. These

cThe starting guesses obtained from the GO calculations were further optimized by fullab initio methods.9 Additional ion pairs were located manually. The lithium ion binding

Figure 1. Schematic anion structures ~a! BOB ~b! MOB ~c! BMB, and ~d!TOP. A-E = lithium ion coordination possibilities. All lithium ion coordina-tion is bi-dentate except E and C2. Please note that the two rings constitutingthe orthoborate anions are twisted ,90 with respect to each other.

A216 Electrochemical and Solid-StatA216energies were calculated as the difference in electronic energies of optimized geom-etries: E = Esion-paird-sEsLi+d + Esaniondd. For the HF calculations the zero point en-ergy corrections to the binding energies were in the range 4-9 kJ mol1 and the totalcorrection of both zPE and basis set superposition error ~BSSE! via counterpoise cor-rections to be applied is shown in parenthesis ~Table I!.

address. Redistribution su150.65.7.77Downloaded on 2013-04-18 to IP results are in accordance with the recent results of Xu et al. thatfound BOB to be less lithium ion coordinating than BMB.16 Allthree anions are less lithium ion coordinating than most traditionalelectrolyte anions, with binding energies no more than ,90% ofthose of TFSI, BF4

, and PF6 ~Table I!.

For the relative strengths of lithium ion association of the differ-ent anions the ab initio methods presented in Table I are in mostcases qualitatively consistent. Changes may occur, though, if thenumeric values are very close and for such cases the values are morereliable the larger the basis set. However, the effect of includingelectron correlation via second order, MP2, perturbation theory ismore delicate. The MP2 correction overestimates the electron corre-lation in the system, and higher order perturbation terms are neededto get comparable results. Another approach is to use an accuratecomposite technique aimed at thermodynamic data, c.f.Gaussian-3,17 that includes both very large basis sets and fourthorder MP perturbation theory, but this is only feasible for very smallsystems.

So far, the data presented do not take any solvent effects intoaccount, which should be especially crucial for liquid electrolytesoften based on high dielectric solvents. By applying the SCRFmethod, the prime effect is the considerable reduction in the abso-lute values of the binding energies, which is due to the large solva-tion energies of the charged species, especially the lithium ion. Theprevious, in vacuum, qualitative ordering of the ion pair bindingenergies; BF4 . TFSI . PF6 . BMB . MOB . BOB, is basi-cally kept in the SCRF calculations, with the major exception of thePF6

anion becoming the weakest coordinating ~Table I!. Vide infra,this is due to a larger solvation energy for the PF6

anion. The MOBand BMB anions changes relative coordination strength, but this iswithin the uncertainty of the calculations. There are also somechanges in the relative order of the different ion pairs for each anion.The most profound is that the B and D positions are more favorablethan in vacuum. The coordination in these ion pairs, however, is alsono longer bi-dentate, but rather monodentate to the carboxyl oxygenatom.

The most unique and important information obtained is that thecoordination site A in Fig. 1 is the strongest lithium ion coordinationsite for the orthoborate anions, whenever a possibility. Indeed, thevery recent first XRD structural characterization of the LiBOB salt18report this type of chelate coordination to be of importance and theassociated bond lengths ~1.94 and 2.02 ! and angle ~89.5! are infair agreement with our obtained values ~1.92 and 93.6, gasphase!. Thus, to further increase the salt dissociation we seek toreduce the lithium affinity of this site or the site itself eliminated asin the BMB anion, where, however, A2 is a possibility. A2 is favoredin vacuum, but in line with the discussion above D becomes themost stable ion pair in the SCRF calculations.

Using the approach of reduced affinity discussed above, we pro-pose the possibility of a P-analogue of the BOB anion: the TOP~tris-oxalato-phosphate! anion ~Fig. 1d!. Our present results showthis anion to be even less lithium ion coordinating than BOB andalso, from the HOMO value, more stable towards oxidation ~TableI!. In addition, the SCRF calculations reveal the TOP anion to be asweakly coordinating as the PF6

anion in strong dielectric media.Apart from the most preferred site ~A! there are three additionalcoordination possibilities for the lithium ion: B, C2: tri-dentate co-ordination to oxygen atoms bonded to the central phosphorous atom,and finally A2: bi-dentate coordination to oxygen atoms from differ-ent oxalate groups ~Fig. 1d!. As for the orthoborate anions, B isfavored in the SCRF calculations, but yet A is the most stable ionpair. In addition, C2 is no longer a tri-dentate structure, but ratherbi-dentate.

Turning back to the TOP anion itself, the question arises if theP-O bonds are strong enough to be chemically stable. Compared to

ters, 8 ~4! A215-A218 ~2005!the length of a standard P-O single bond of 1.54 , with a strengthcomparable to a C-C single bond,19 the present P-O bonds are,0.17 longer, and should thus be considerably weaker. However,the tris@1,2-benzenediolato~2!-O,O#phosphate ~TBP! anion of

ecsdl.org/site/terms_usebject to ECS license or copyright; see

O [e

Ea~full!/

sdd18

77

868

95958

178

7

4

44

b

8

e LetHanda et al.20 with a very similar six-coordinate P atom center hassix comparable P-O bond lengths of ,1.7 23 and this anion wasfound to be electrochemically stable up to ,3.7 V vs. Li+/Li0.20-22

A profoundly important aspect of new lithium salts for electro-lytes is the electrochemical window within which they can be usedsafely. The orthoborate salts have been shown to have excellentstabilities: .4.5 V vs. Li+/Li0,7 arguably in some cases solvent de-composition limited. Both Ue et al. and Barthel et al. have shownthat for a family of lithium salts for electrolyte purposes the relativeelectrochemical stability can be evaluated using the HOMO energiesof the anions.23,24 In general, the HOMO energy is reflecting thestability of the system vs. oxidation. From Table I it is found that theHOMO values are similar for all three orthoborate anions: ,8.5 eV.From inspecting the MO coefficients and by visualizing the HOMOwe find the HOMO to be heavily delocalized, but the main contribu-tors are the 2p orbitals of the carbonyl oxygen atoms for all threeorthoborate anions. The TOP anion has an even lower HOMO valuedue to the extra ring, that further delocalizes the HOMO, and alsofor TOP the 2p orbitals of the carbonyl oxygen atoms are maincontributors. Using our own and experimental data presented byUe20 we find a rough correlation Eox = s42-HOMOd/8.3 for the in-organic anions. Using this equation to calculate Eox we obtain apositive agreement with the experimental observations for theorthoborate anions ~Table I!. Note that the instability of the PF6

andBF4

anions, or the orthoborate anions, toward decomposition viahydrolysis, and for the former two release of F, is inherently notreflected by this measure.

Conclusions

Table I. Binding energies DE kJ mol1 (zPE + BSSE corr.), HOM

Ion pairs

DEaHF/

6-31Gsdd18

DEaHF/

6-31+Gsdd18

DMP26-31G

BF4

Li-F ~bidentate! 647 s35d 593 s9d 67Li-F ~tridentate! 637 s35d 579 s8d 67BOBLi pos A 539 s23d 515s10d 55Li pos B 474 s17d 453 s7d 49Li pos C 472 s18d 449s10d 50MOBLi pos A 554 s16d 530s10d 56Li pos B 491 s13d 470 s7d 52Li pos C 507 s15d 483s11d 53Li pos D 504 s13d 482 s8d 52Li pos E 515 s17d 488s10d 54BMBLi pos A2 566 s22d 539s10d 59Li pos C 540 s18d 515s11d 56Li pos D 521 s21d 499 s8d 53TFSILi-O ~bidentate! 606 s23d 588s15d 62PF6

Li-F ~tridentate! 602 s29d 559s10d 64TOPLi pos A 491 s21d 472s13d 51Li pos A2 455 s19d 435s10d 48Li pos B 430 s15d 412 s8d -Li pos C2 463 s19d 440s12d 50

a Binding energies: DE = Esion-paird sEsLi+d + Esaniondd.b No minimum energy geometry was obtained.

Electrochemical and Solid-StatBy using standard ab initio methods, both the electrochemicalstability and lithium ion coordinating properties of the new orthobo-rate anions BOB, MOB, and BMB have been calculated. Thelithium ion affinity, obtained via the most stable 1:1 ion pairs, was

address. Redistribution su150.65.7.77Downloaded on 2013-04-18 to IP found to be on par or better than most anions used today, both forpolymeric as well as for liquid ~or gel-type! electrolyte application.Furthermore, we use our present data to suggest a new anion possi-bly suitable for all electrolytes and from the final results the LiTOPsalt is an alternative worthy of synthesis consideration ~see below!.

AcknowledgmentsThis work was supported with grants from ngpannefreningens

Forskningsstiftelse and the Swedish Research Council ~VR!. Thecomputations were performed at the Center for Parallel Computers~PDC! at KTH, Stockholm through a computational grant fromSNAC. The authors acknowledge Prof. C. A. Angell for fruitfuldiscussions.

Chalmers University of Technology assisted in meeting the publicationcosts of this article.

AppendixDuring the acceptance process of this paper, we became aware, by the kindness of

Dr. Ulrich Wietelmann of Chemetall GmbH, of a very recent experimental study on theLiTOP salt.25 The conclusions of that work of course provide essential comparison datafor our present computations: the TOP anion and its lithium salt are indeed obtainableby synthesis, the anion has a wide voltage window, and the salt is a potential candidatefor nonaqueous battery electrolytes. All those conclusions are in line with our data. Alsoa LiTOP patent exists.26

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HOMOMP2~full!/6-31G~d! Calc. Eox

Exp. Eox7,27,28

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3325

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A218 Electrochemical and Solid-State Letters, 8 ~4! A215-A218 ~2005!A218 address. Redistribution su150.65.7.77Downloaded on 2013-04-18 to IP ecsdl.org/site/terms_usebject to ECS license or copyright; see