nanomaterials Review Nanostructured MnO2 as Electrode Materials for Energy Storage Christian M. Julien * ID and Alain Mauger Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Unité Mixte de Recherche 7590, Sorbonne Universités, 75005 Paris, France; [email protected] * Correspondence: [email protected]; Tel.: +33-144-272-443 Received: 14 October 2017; Accepted: 5 November 2017; Published: 17 November 2017 Abstract: Manganese dioxides, inorganic materials which have been used in industry for more than a century, now find great renewal of interest for storage and conversion of energy applications. In this review article, we report the properties of MnO2 nanomaterials with different morphologies. Techniques used for the synthesis, structural, physical properties, and electrochemical performances of periodic and aperiodic frameworks are discussed. The effect of the morphology of nanosized MnO2 particles on their fundamental features is evidenced. Applications as electrodes in lithium batteries and supercapacitors are examined. Keywords: energy storage and conversion; nanomaterials; MnO2; lithium batteries; supercapacitors 1. Introduction In recent years, intense Research & Development input on nanotechnology has delivered nano-objects (particles with size ≈ 100 nm or less) that possess a rich combination of physical properties, inasmuch as they differ from those of the bulk and depend on polymorphism, morphology, size of particles, size distribution, coating, and the precursor used in the synthesis [1]. With the need for renewable energies, these nano-substances have undergone extensive research, in order to develop new systems that can be used for energy storage and/or conversion. Among them, transition-metal oxides including TiO2, MnO2,V2O5, etc. are stable and robust materials with tunable properties offering large surface areas. In the early ages, mineral manganese dioxides (MDOs) were used as black pigments for rock-art painting in paleolithic caves of the Magdalenian culture [2]; they can be considered as the first nanomaterials used up until then by human civilization. Today MnO2 is an important functional metal oxide, which is technologically attractive for applications in different fields such as catalysts [3–5], absorbent of toxic metals [6], ion-sieves, molecular-sieves [7], artificial oxidase [8], component of the dry cell (Leclanché cell) [9], inorganic pigment in ceramics, electrodes for electrochemical batteries (lithium, magnesium, sodium) [10–13], and electrodes for supercapacitors [14,15]. MnO2 has also been widely used in Duracell (alkaline) based barriers, photocatalytic activities, and electrolysis. Owing to its ability to absorb toxic ions, MnO2 has been also found to have important applications in water-cleaning [16,17]. MDOs are non-stoichiometric compounds, because of inevitable structural water molecules that are physisorbed. The engineering of manganese oxides used for energy storage and conversion has become more and more important to the point where a huge number of works is devoted to these materials. The great interest of MnO2 as an inorganic material in the battery industry is due to its theoretical −1 −1 −1 capacity (308 mAh·g comparable to 270 mAh·g of LiCoO2) and capacitance (1370 F·g ), natural abundance, low cost, and low toxicity [1]. MDOs crystallize with various morphologies and crystallographic forms including the a-, b-, g-, d- #-, l- and R-polymorphs, which are naturally Nanomaterials 2017, 7, 396; doi:10.3390/nano7110396 www.mdpi.com/journal/nanomaterials Nanomaterials 2017, 7, 396 2 of 41 Nanomaterialsabundance,2017 low, 7, 396cost, and low toxicity [1]. MDOs crystallize with various morphologies2 ofand 42 crystallographic forms including the α-, β-, γ-, δ- ε-, λ- and R-polymorphs, which are naturally occurring minerals such as hollandite (2 × 2), pyrolusite (1 × 1), nsutite (1 × 1)/(1 × 2) with hexagonal occurring(hex.) structure, minerals birnessite such as hollandite (1 × ∞), akhtenkite (2 × 2), pyrolusite (dense stack), (1 × 1), spinel nsutite (1 (1 ×× 1),1)/(1 and× ramsdellite2) with hexagonal (1 × 2), (hex.)respectively, structure, where birnessite (m × n) (1 denotes× ¥), akhtenkitethe tunnel dimension. (dense stack), The spinel polymorphism (1 × 1), and is ramsdellitedue to the different (1 × 2), × respectively,ways of linking where the (MnOm n6 )octahedral denotes the architectonic tunnel dimension. units throug The polymorphismh corner- or edge-sharing is due to the that different show waysvariations of linking in the the chain MnO and6 octahedral tunnel structures architectonic (see unitsFigure through 1) [18]. corner- For example, or edge-sharing the (3 × that3) tunnel show × variationsstructure of in todorokite-type the chain and tunnelMnO2 structures(octahedral (see molecular Figure1 )[sieve18 ].labeled For example, OMS-1) thehas (3a pore3) tunnelsize of structureabout 6.9 ofÅ [7]. todorokite-type Note that the MnOhollandite,2 (octahedral cryptomelane, molecular and sieve coronadite labeled minerals OMS-1) that has are a porestructurally size of aboutrelated 6.9 to Å (2 [ 7×]. 2) Note tunnel that materials the hollandite, have water cryptomelane, and different and cation coronadite contents minerals such as that Ba are2+, K structurally+, and Pb2+, 2+ + 2+ relatedrespectively to (2 × [19].2) tunnel Conseque materialsntly, properties have water of and MDOs different depend cation stro contentsngly on such the ascrystalline Ba ,K , structure, and Pb , respectivelyparticle size, [and19]. morphology. Consequently, The properties crystallographic of MDOs data depend of some strongly MDO compounds on the crystalline are summarized structure, particlein Table size, 1. and morphology. The crystallographic data of some MDO compounds are summarized in Table1. FigureFigure 1.1. RepresentationRepresentation ofof thethe differentdifferent MnOMnO22 frameworksframeworks characterizedcharacterized byby theirtheir tunneltunnel ((mm ×× n)) structures.structures. ((a)) pyrolusitepyrolusite (1(1 × 1);1); ( (bb)) ramsdellite ramsdellite (1 (1 ×× 2);2); (c ()c )hollandite hollandite (2 (2 × ×2);2); (d ()d birnessite) birnessite (1 (1× ∞×);¥ (e);) (romanechitee) romanechite (2 × (2 3);× and3); and (f) spinel (f) spinel (1 × (1 1).× Reproduced1). Reproduced with with permission permission from from [18]. [ 18Elsevier,]. Elsevier, 2004. 2004. SinceSince MnOMnO22 was introduced as aa depolarizingdepolarizing elementelement inin zinc-alkalinezinc-alkaline cells,cells, aa majormajor stepstep waswas realizedrealized withwith thethe utilizationutilization ofof naturalnatural andand synthesizedsynthesized materialsmaterials suchsuch asas electrochemicallyelectrochemically (EMD), chemicallychemically (CMD),(CMD), andand heat-treatedheat-treated (HTMD)(HTMD) preparedprepared MnOMnO22.. All thesethese frameworksframeworks areare generallygenerally classifiedclassified asas membersmembers ofof thethe nsutitensutite group:group: thethe gγ-MnO-MnO22 andand #ε-MnO-MnO22 phases,phases, distinguisheddistinguished byby thethe qualityquality ofof theirtheir X-rayX-ray diffractiondiffraction diagramsdiagrams [[9].9]. DirectDirect synthesissynthesis ofof EMDEMD oror CMDCMD resultsresults inin structuralstructural defects:defects: (i)(i) thethe “De “De Wolff Wolff defects” defects” (denoted (denoted Pr) arePr) intergrowthare intergrowth of pyrolusite of pyrolusite in the ramsdellitein the ramsdellite matrix andmatrix (ii) theand micro-twinning(ii) the micro-twinning defects (denoted defects Tw). (denoted Figure 2Tw). shows Figure the degree 2 shows of micro-twinnings the degree of asmicro-twinnings a function of theas a pyrolusitefunction of intergrowththe pyrolusite in intergrowth synthesized in CMD, synthesized EMD, andCMD, HTMD EMD, manganeseand HTMD dioxides.manganese In dioxides. commercial In commercia EMD powders,l EMD Pr powders, and Tw are Pr closeand Tw to 45–55%are close and to 80–100%,45–55% and respectively. 80–100%, Beingrespectively. relatively Being cheap, relatively EMD materials cheap, EMD are widely material employeds are widely as electrodes employed inprimary as electrodes alkaline in batteriesprimary andalkaline supercapacitors. batteries and EMD supercapacitors. powders with manganese-cakeEMD powders microstructurewith manganese-cake (predominantly microstructureg-MnO2 phase)(predominantly deliver a γ specific-MnO2 dischargephase) deliver capacity a specific of 280 di mAhscharge·g−1 capacityusing 9 of mol 280·L −mAh·g1 KOH−1 electrolyteusing 9 mol·L [20−].1 AnKOH extensive electrolyte review [20]. An devoted extensive to electrolytic review devoted MnO to2 haselectrolytic been published MnO2 has recently been published [21]. Therefore, recently we[21]. simply Therefore, direct we the simply readers direct to this the review readers for to specificthis review properties for specific of EMD, properties and attention of EMD, in and the present work is thus focused on other syntheses and related MnO2 in the other a- and b-phases [21]. Nanomaterials 2017, 7, 396 3 of 41 Nanomaterials 2017, 7, 396 3 of 42 attention in the present work is thus focused on other syntheses and related MnO2 in the other α- and β-phases [21]. For the same reason, we did not detail any discussion on the MnO2-based Forsupercapacitors, the same reason, because we did a 43-page not detail review any discussion on them, including on the MnO experimental2-based supercapacitors, aspects and discussion, because a 43-pageprospective, review has on been them, recently including published