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Many extensions of the Standard Model of particle physics suggest thatneutrinos should be Majorana-type fermions, but this assumption is difficult toconfirm. Observation of neutrinoless double-beta decay ($0\nu \beta \beta$), aspontaneous transition that may occur in several candidate nuclei, would verifythe Majorana nature of the neutrino and constrain the absolute scale of theneutrino mass spectrum. Recent searches carried out with $^{76}$Ge (GERDAexperiment) and $^{136}$Xe (KamLAND-Zen and EXO-200 experiments) haveestablished the lifetime of this decay to be longer than $10^{25}$ yr,corresponding to a limit on the neutrino mass of 0.2-0.4 eV. Here we report newresults from EXO-200 based on 100 kg$\cdot$yr of $^{136}$Xe exposure,representing an almost fourfold increase from our earlier published datasets.We have improved the detector resolution at the $^{136}$Xe double-beta-decayQ-value to $\sigma$/E = 1.53% and revised the data analysis. The obtainedhalf-life sensitivity is $1.9\cdot10^{25}$ yr, an improvement by a factor of2.7 compared to previous EXO-200 results. We find no statistically significantevidence for $0\nu \beta \beta$ decay and set a half-life limit of$1.1\cdot10^{25}$ yr at 90% CL. The high sensitivity holds promise for furtherrunning of the EXO-200 detector and future $0\nu \beta \beta$ decay searcheswith nEXO.

Search for Majorana neutrinos with the first two years of EXO-200 data